I will second what Kelley said - I just bought the same Sigma 18-125.
F/3.5-5.6 refers to the range of apertures the lens is capable of. Aperture, which is measured in F-stops, if how large an opening the lens has to let light in to strike the sensor or film. The DC refers to Sigma's Digital Camera series of lenses, which are specifically designed to work with the new crop of digital SLR cameras like the Digital Rebel, D20, or Nikon D70.
Kelley is certainly right that you can easily get away without an external flash at WDW. But if you do decide to get one for use at home, a Canon Speedlight 220EX is the one I have. It costs about $130, is about 10 times brighter than the built-in flash, runs on 4 AA batteries, and is totally automatic. It also has a built-in infrared light to allow the camera to focus in low light situations.
But some other inexpensive accessories you might consider are a mini-tripod and the infrared remote control. The remote runs about $25 and will let you trip the shutter on your camera without actually touching it, thus eiminating shaking that would blur a long-exposure shot. The tripod will run anywhere from $15-$30 depending on size, brand, and where you buy it, and wil allow you to put the camera down on a table or trash can to get long-exposure shots at night.
And yes, I carry my camera around with me everywhere when at WDW. For a little more detail, you can check my page on Theme Park Gear:
Will, thanks for the great suggestions. I'm glad I have until October to save $$$ to buy all these gadgets! Do you purchase your camera supplies from BH Photo online, or from a retail store? I looked for a Sigma lens on their site and couldn't find one, but I think I was searching the wrong way. In another post I was reading, I was surprised how many differing opinions people have about camera lenses. Pretty creative little carrying system you've devised. I guessed you were an engineer before I finished reading your web site! Thanks again.