I have a Latitude D820 (currently on it), go with the N-Series if you want to run vista, that way you dont pay for XP. Get the 2gb of ram, 7200rpm hdd upgrade, the screen upgrade (1680, not 1900.. cant see crap on the 1900), second battery if you travel. They are really great laptops, 5 hours battery time when developing on low power mode. We have about 20 of them at work, every member of the support team has one, a few of the developers do for personal use, and the network admin has one for personal use. Not had any problems with them yet.
-- Real programmers don't comment their code. It was hard to write, it should be hard to understand.