My $.02. When I upgraded to my Phenom system from my 939 system I got 4GB because it's so cheap it just made sense to get 4GB instead of 2GB. In all honesty, I doubt very much that 4GB vs. 2GB matters a whole lot if you're only gaming, without other things going on in the background. But then again I'm on XP, not sure if Vista is that different or not.
Slightly off topic, does Vista32 see all 4GB, or is it still less due to the 32bit limitation? XP Pro only sees 3.2GB for me, I'm guessing due to the 512MB on my video card. Still not sure where that other ~300MB is. Anyway, I wasn't sure if Vista32 had some kind of upgraded tech to let it use all of the installed 4GB, or if there is no way around the 32bit limitation other then going with a 64bit OS (and of course the 36 bit address extension that some of the server stuff uses, but that's not really relevant here).
Anandtech editors, there's a nice article idea... test a handful of popular games at a few different resolutions on XP, Vista32, and Vista64 with 1GB,2GB,4GB, and 8GB of memory. I think we'd all be interested in the results.