im running vista 64bit and i have had no compatability problems. There are a FEW programs that have slight issues, like coretemp for example... however all these issues have been resolved already with little fixes... i jsut had to google vista 64bit coretemp and i found a program that i run b4 i open it so that it works... thats about the most complicated compatability issue i have run across so far... and it took all of a minute and a half to solve.
as far as the RAM, id suggest 4 gigs. i really cant see it being too much of an issue with 2, but it depends on what games you play or if you into any video editing or stuff like that.
most of my games that i run on all high settings use about a gig of ram, give or take 250mb, and i use about 750-1000MB of ram just idling (thats with AVG, steam, xfire, and vent running). So if u do the math ur fine with 2 gigs for most everything.
if you enjoy gaming and are planning on getting a GPU and CPU to run them on high settings, i would suggest 64bit with 4 gigs of ram.
its always nice to know that its there if you need it
... and in my case i like to have a game playing and about 3 tabs of firefox open and stuff... multitasking is great