I think this is more of a matter of personal preference really. One thing is for sure, I say 1gig at minimum for any hardcore user. 1gig is normally more then enough for anyone... But, having $$$ and additional slots on your MOBO for another GIG won't hurt a bit. The more RAM the better I always say.
Both of my PC's have 2 GIG.
ESTIMATED USAGE in %
Dell 8300 - P4 3.4GHz / X800XT Pe / 2gig (4x512MB) Corsair Value DDR400
(Normal Usage = 28-32% used / Heavy Usage-Gaming 35-45% used) - This PC has TONS of things running at the same time. Lot's of neat little GUI programs, windowblinds, object desktop, object dock, ETC.. Lot's of other stuff in taskbar... Too much actually... THe highest i've ever got this one to was 59% - But that was more of an experiment that anything else... I just kept loading all kinds of applications. Photoshop, web, winamp, dvd movie, email, word, excel, studio 8, running AV scan, running spyware scan, - and tons of background stuff... funny thing is, it was slow... but it was very usable. Much better then alot of PC's i've used...
FX-55 - DFI - Lanparty SLI-DR - (2) eVGA 6800u SLI - CORSAIR XMS DDR400 (4x512MB) TWINXLPRO's 2.2.2.5
(Normal Usage = 15-20% used / Heavy Usage-Gaming 30-35% usage) - This PC has very little running in the background, only the absolute essentials... Firewall, AV, OS services, Normal stuff...
SO I guess really all anyone needs is 1 gig...