Depending on what games you actually play will determine if 2gb is going to be sufficient for you in Vista. BF2 in XP needs 2gb in order to run its smoothest. Company of Heroes and Supreme Commander are others.
As i said before.. the OS uses half a gig alone outside of the apps that it caches in ram compared to XP. With Superfetch enabled... there just isn't any possible way for me to play Supreme Commander in Large maps, with Lots of units, while having my 2gb ram being cycled from cached apps to caching game files in real time to achieve the same performance as if I had more headroom like 4gb to start with. There wouldn't be any need of cycling cached data in ram in this case. Let's also not forget that you will eventually be using the pagefile once you run out of Physical memory.
Who knows how much ram UT3, Crysis, Alan Wake, BIA: Hells Highway.. will use.