Generally speaking, Win XP Pro and Norton Antivirus are memory whores no matter what machine I've ever used, but once they are fully loaded, I can load three scenes of 3d Max effortlessly (350,000 k's each) and the computer can work on those overnight without issue. I have 1.5 gigs of ram and a 2.2 ghz Athlon 3000+ processor.
Build your new PC geared toward upgradability, otherwise you're shooting yourself in the foot, as people have already said.
