in the end it also depends on what its going to be used for.
Basic internet tasks, office crap and so on. Modest video card, a pile of ram and a half decent processor.
Hardcore FPS games, you'll want the biggest machine you can afford, which will give you a few years worth of computer.
If its going to run windows, toss in some extra RAM for good measure as vista and 7 will both pretty much sink your machine if you try to do more than one thing at a time.
Extra Ram? Definitely! But Vista and Win7 sink multitasking? What kind of processor are you using? I've got a old dual core Athlon 64 X2 running at 3.0 Ghz and under either of those OS' I have just as good response as XP.
Of course I'd recommend at least 2Gigs of Ram (or better 4Gigs) because all NT kernel versions of Windows (basically everything from NT/2000 to Win7) can use the extra memory to buffer a lot of different hardware operations.
You want slow (or non functional) load XP onto a machine with less than 1Gig of memory and then open up a few dozen browers tabs or windows.
Your response will slow to a crawl or crash. Under Vista or Win7 (with enough memory) I've never had this happen. And, when you tell Vista or Win7 to shut down, they do so promptly. XP, if there's any processes running tends to sit there for a while like an Alzheimer paitent till it finally gets around to closing everything down.
Don't believe everything you hear about Vista or Win7. Pretending that either isn't a polished up version of XP is like pretending that WinME wasn't Win98 with a few minor changes.
