If you're going for a PC I'd suggest, as a minimum, these specs:
512MB RAM, possibly even 1GB
120+GB hard drive, for all the porno and music. The bigger, the better, basically.
15" or bigger screens are awful nice to have. Preferrably not widescreen, I can't stand wide displays unless they're really big (17"+, but you probably won't find one in your budget).
CPU.. dunno, you don't need anything beyond 2GHz.
Useful, and probably standard on most laptops nowadays:
Built-in Ethernet and possibly wireless networking if you need that.
A good number of USB ports, say, 4.
Those figures are what I would base my next purchase on, if I weren't still happy with this laptop that I bought three years ago for under $1k.
You won't need any special graphics, so any crappy integrated video (Intel, SIS, whatever) will do just fine.
As for Vista, I don't care for it myself, I'm still running Windows 2000 and am perfectly happy with it.
From here on it's about how much battery life you need, and if you're going to be lugging it around a lot you're going to want it to weigh less than 8lbs.