If you're like me and like to keep a bunch of apps running at the same time the extra memory will aslo benefit you as the OS won't have to swap to disk as often. I have Win7 x64 with 8 gig and I'm quite pleased with it. I originally had 4 gig in it and I've noticed it's a lot smoother switching between apps with the extra 4 gig.
6gb will do as i have. Win 7 64bit requires 1gb just to run itself. You might have 1/2 gb just to run extra apps in the background like antivirus, antispyware, defragmenter, registry fixers, updaters.... so 2gb is enough to run the system with your apps in the background... you would want an extra 4gb to run games, burning software, music production, video editing and stuff. 6gb will do just fine... unless you plan on playing games and hearing music and burning software all at the same time, then go with 8gb.