If choosing between, the SB-400 and the SB-600, I would probably go with the SB-600 for the extra $75. If later down the road, you want to start doing off-camera flashes, then just add the SB-800 or SB-900, since I don't believe that the D60 has a commander mode which can remotely fire a flash. I am not sure if you can remotely control an SB-400 either.
Either the 400 or the 600 should give you the capabilities for making better photos, but the 600 may give you more options down the road. If you can afford the extra $75 or so, then go for the 600.
I pulled these off of
Amazon for you.
From the Manufacturer
Nikon announces the Nikon Speedlight SB-400, an intelligent and versatile external flash unit that provides i-TTL flash operation in a super-compact and ultra-lightweight body at an affordable price.
With its powerful guide number of 21, the SB-400 is an ideal flash unit for beginners in digital SLR photography, but it is also useful for hobbyists and professionals looking for a handy i-TTL flash unit offering an effortless bounce flash function.
The SB-400 has been specifically developed to provide photographers with an entry-level external flash unit which is compatible with i-TTL flash operation. When combined with cameras that are compatible with the Nikon Creative Lighting System, the SB-400 is controlled in i-TTL and D40's M (manual) mode.
The SB-400's flash head can be tilted in 4 steps--horizontal, as well as 60, 75, and 90 degrees--and has a flash shooting distance range of between 60cm to 20m (depending on the camera's ISO setting). Slow Sync, Red-eye Reduction and other flash modes can also be set on the camera body, enabling users to enjoy brighter and more natural lighting for better flash pictures.
Product Description
The SB-400 gives photographers the opportunity to enjoy brighter and more natural lighting for images taken using a flash. It is designed to suit entry-level digital SLR users while also suitable for more experienced users. Those who have a camera within the D2 series will also benefit from the convenient portable flash unit. The SB-400 offers a guide number of 30 (ISO 200, m), and effortless bounce flash function and is controlled in i-TTL and the D40's M (manual) mode. Flash shooting distance range from 2 and up to 66 feet Electronic construction - Automatic Insulated Gate Bipolar Transistor (IGBT) and series circuitry Guide number - (at 18mm zoom-head position, 20 degrees C/68 degrees F) 30m per 98.4 feet Angle of coverage - Light distribution covers an 18mm lens when mounted on a Nikon DX format digital SLR camera (27mm lens when mounted on an F6 35mm SLR) Flash shooting distance range - 2 to 66 feet (Varies depending on the ISO sensitivity, zoom-head position, and lens aperture in use) Flash mode - i-TTL, M (manual with the D40 only) Flash exposure control set on the camera - Slow-sync, red-eye reduction, red-eye reduction in slow sync, rear-curtain sync flash, FV lock flash, manual flash (with Nikon Creative Lighting System digital and 35mm SLR cameras) Bounce capability - Flash head tilts up to 90 degrees with click-stops at 0, 60, 75, 90 degrees Power Source - Two AA batteries (1.5V or lower) Flashes/Recycling time at full output - Alkaline-manganese (1.5V) 140/3.9 sec, Lithium (1.5V) 250/4.2 sec, Oxyride (1.5V) 150/3.1 sec, Ni-MH rechargeable (1.5V, 2600 mAh) 210/2.5 sec Lights up when the SB-400 is recharged and ready to fire. Blinks to indicate insufficient light, insufficient battery power and incompatibility of camera with SB-400 Flash duration - Approximately 1/1300th second at full output Mounting foot lever lock - Provides secure attachment to the camera accessory
From the Manufacturer
Nikon introduces the Speedlight SB-600, a compact flash packed with impressive functionality inherited from the renowned SB-800. It serves as a solid base for the Nikon Creative Lighting System when used in combination with Nikon D70 or D2H digital SLRs.
When used in combination with Nikon's latest digital SLRs, the D70 or D2H, the SB-600 supports Advanced Wireless Lighting as a remote Speedlight that can be positioned as one of multiple remote flashes to significantly extend creative possibilities. To assist in such situations, the Speedlight SB-600 also has two Ready Lights located on both the left and right of the front that can be easily seen from a distance. The SB-600 can also emit an audible alert for further assistance.
Auto FP High-Speed Sync enables the use of Fill-Flash even in bright daylight with wide aperture settings for the achievement of outstanding results and new creative possibilities, including the effective blurring for of out-of-focus background elements. An optional color filter set SJ-1 is also available, allowing imaginative shooting to be pushed even further.
White balance is optimized through the use of flash color information obtained by the Speedlight. The SB-600 also features FV lock, which gives the user the convenience of being able to maintain the measured flash value while recomposing the shot to achieve the appropriate flash output for the subject.
It also offers uncomplicated operation made simpler by an easy-to-view LCD with only six buttons, each of which is backlit and simple to understand.
The Nikon Speedlight SB-600 also delivers plenty of style, both in its compact dimensions and a triangular motif consistent with the Nikon D-SLR theme.
The Nikon Speedlight SB-600 comes complete with a Speedlight stand and a soft case, while two flash cables (the SC-29 and SC-28) and the Color Filter Set (SJ-1) are available as options.
Major Features of the Nikon SB-600
* Supports the Nikon Creative Lighting System.
* Supports i-TTL (for automatic balanced Fill-Flash), D-TTL, TTL, Manual.
* Easy-to-view LCD with 6 simple-to-understand backlit buttons.
* Guide Number of 30/98 [ISO100,m/ft], 42/138 [ISO200,m/ft] (at 35mm zoom).
* Auto zoom of 24 to 85mm, extendable to 14mm with built-in wide-flash adapter.
* Same Wide area AF Assist Illuminator as the popular SB-800.
* Advanced Wireless Lighting available with D2H or D70 when used as a remote.
* Auto FP High-Speed Sync (with D2H only); use with fast shutter speeds achieves effective lurring of out-of-focus background elements.
* Flash Color Information - optimized white balance makes use of data from the Speedlight.
* FV lock holds flash value, enabling recomposition prior to shooting.
* Accurate i-TTL flash control achieved through flash exposure monitoring control.
* Design consistent with those of next-generation D-SLRs, with a triangular design element on the top that complements the D-SLR motif.
* Supports optional Color Filter Set (SJ-1).
* Ready light located beside the LCD on the back panel - highly visible even when used as a remote.
Product Description
When used in combination with Nikon's latest digital SLRs, the D70 or D2H, the SB-600 supports Advanced Wireless Lighting as a remote Speedlight that can be positioned as one of multiple remote flashes to significantly extend creative possibilities. To assist in such situations, the Speedlight SB-600 also has a readylight located beside the LCD on the rear panel that can be easily seen from a distance. The SB-600 can also emit an audible alert for further assistance. Easy-to-view LCD with 6 simple-to-understand backlit buttons Guide Number of 30/98 [ISO100,m/ft], 42/138 [ISO200,m/ft] (at 35mm zoom) Auto zoom of 24 to 85mm, extendable to 14mm with built-in wide-flash adapter Advanced Wireless Lighting available with D2H or D70 when used as a remote Auto FP High-Speed Sync (with D2H only); use with fast shutter speeds achieves effective blurring of out-of-focus background elements Flash Color Information - optimized white balance makes use of data from the Speedlight FV lock holds flash value, enabling recomposition prior to shooting Design consistent with those of next-generation D-SLRs, with a triangular design element on the top that complements the D-SLR motif Requires 4 AA Batteries (Alkaline or Rechargeable NiCad or NIMH) for use
cool1: 
) that I just purchased yesterday (online, so I don't actually have it in my possession yet), but I would like something a little less harsh than the pop-up flash. 
