Considering you don't say how much RAM you currently have, or what you do with your computer, it's pretty much impossible for someone to give you an answer.
2GB of RAM works out well for me for running some games like Oblivion or Civ4. I also run SQL Server 2005 (runs all the time, even if I'm not using it), and I do software development with this computer (but not when I'm playing games, obviously). I found with 1 gig of RAM, I'd want to make sure that everything was shut down before firing up a game, and even then, it was swapping. Heaven forbid that my wife was logged into XP at the same time; I'd have to go log her out first, and then fire up the game.
With Vista, more RAM is even better, but I still don't find myself getting too close to my 2GB of physical memory, so I don't plan on plunking in more memory. Vista is currently using about 1.3GB of memory, with Outlook 2007 and a few browser windows open. Vista handles memory "differently" than XP, in my experience. It's more willing to use all of your resources it can, but it doesn't seem to cause problems when other applications need them later.
Clint