IMO Photoshop would run at its best with TWO SEPARATE hard drives, partitioning a single drive will probably slow down Photoshop (varies depending on what background operations the OS is running). Remember VISTA is a resource hog and running it on the same drive as the scratch disk will defeat the purpose.
Exactly correct about the scratch disk thing. The advantage is from using two different physical drives, and partitioning a single disk to use one partition as a scratch disk will lower performance.
To the OP... I usually tell people to only buy a laptop if you need one. They are, without exception, more expensive, slower, less reliable, more expensive and more difficult to repair, and less expandable than a desktop unit. If you need one, great, but I wouldn't get one just for the heck of it, especially if you're serious about photography (or gaming or performance in general.)
Furthermore, most laptop screens have fairly small angles of view - in other words, you have to look straight at it, as soon as you're a little too high or low or to the side, the colors change dramatically, which is obviously not what you want for photography! See if you can find one that lists a decent angle of view. Or, look into getting a decent standalone LCD screen to use at home to plug into it.
Controversial opinion section, don't read if you're easily offended:

Having supported hundreds of laptops in my day, I wouldn't buy a Dell laptop under any circumstance. I haven't used any but I've heard nightmares about Acers, also. I am ambivalent on Compaq/HPs, Toshibas, and Gateways. I love Lenovo (formerly IBM) Thinkpads. I also have zero interest in a Mac, I much prefer the flexibility that I get from a Windows-based system, and none of those ridiculous one-button mice.

To say nothing of the frequent OS upgrades at $129 a pop. Don't get me wrong, I hate MS as much as the next (honest!) but Apple is easily as evil, closed, and anti-competitive at MS, probably more so.
You also can often get Thinkpads with Windows XP instead of Vista - but you'll have to move quickly as XP is going away forever in a month or so. However, it's not worth getting more than 3 gigs of memory with XP - but really, that's plenty for the vast majority of people IMHO. (Even with Vista, it's a good amount, but you will be more likely to see the benefits of more memory with Vista due to it being a larger OS, but you'll need the 64-bit version in order to address more than 4 gigs (which probably won't be an issue with a laptop.)
One other thing - a 17"-screen laptop sounds good, but such a laptop is going to be
big and
heavy - make sure that is something you're comfortable with before buying. Maybe check a few out in a
Best Buy or similar store. As I mentioned before, you can always go with a smaller laptop and buy a larger monitor to use with it at home for photo editing.