In my observations, there are two things that slow down Visual Studio on a box. #1 is having too many files open, sucking up all your RAM. #2 is loading files off the hard drive in order to compile. In a small project this isn't a big deal, but as the project grows you'll find yourself loading more and more files to compile.
As a result, the biggest things I look for in a development machine is RAM, Disk speed, and then CPU. Some may say that RAM and Disk speed can be the other order, though that isn't my experience with VS.
I'd say look for a laptop with at least 4gb of RAM, then for as fast a hard drive as possible. Preferably a solid-state drive, but that may not be in your budget. Depending on what you're working on, lots of disk space may easily be offset by the speed of the drive. Remember, you can always delete a project and then check it out from source control again later (you are using source control, right?). From there any of the Intel Core 2 chips over ~1.8ghz should give you fairly good CPU performance.