I'm running W7RC-32 right now on an older test mule:
A64 3500+ (Newcastle); Abit AV8 (VIA K8T800 pro); 1gb of PC3200; generic Radeon 9550 AGP card, and an ATA100/160gb HDD.
With all usual background apps (ASW,AV, etc.), a couple unusual ones (Squeezecenter), and Firefox running, it's eating around 450 mb. Not bad, and not that much more than XP-SP3.
For all pedestrian uses, W7 runs adequately on this older system, with some occasional stumble when the multitasking heats up or more complex video is rendered. However, I don't think I would count on it for any serious video editing, as the CPU is not loafing most of the time and there are plenty of bottlenecks elsewhere.
W7-64 might present an entirely different outcome.
In a newer system, I think anything 2gb RAM or more should be more than fine for the typical user. The GPU appears more important with all the Aero effects going on. A 64 bit version doesn't make much sense with under 4 gb.
I could live with W7, based on what I'm seeing.
*****
RIP Les Paul