First, if you are a big gamer and power user, you shouldn't be considering Vista at this time. The performance hit for games on Vista conpletely negates any good you might get from using Vista. It's pretty sad when state of the art computers are playing games at 25FPS using Vista, but the same hardware gets 100+ FPS on XP.
Second, 2GB of ram or 4GB of ram makes no difference. XP does have the ability to allocate 4GB of RAM, kinda. It cannot allocate more than 2GB of RAM for the kernel space, and 2GB of RAM for the application space. The 64-bit version does not have this limit. Buying 4GB of RAM really is the most you can hope to use on a 32-bit version of Windows.
If you are trying to be somewhat 'future-proof' 64-bit is the way to go IF you do your homework and make sure ALL of your hardware and software either has a 64-bit version, or runs well with 32-bit emulation. If you want to be 'now-proof' your better off with 32-bit because you can be guaranteed that your hardware does have drivers, and your games will run.
A year ago I was running Windows 2003 Standard 64-bit. I didn't think it was a problem because I had all the drivers and had no intention of running any software on it. The more software you install the more you open yourself up to security holes. Not a problem until I wanted to do VERY basic things. You want to defrag? Good luck! The 64-bit version was an arm and a leg. The 32-bit version was $99.99. Guess what happened when I tried to repartition a hard drive on it? Or tried to mess with the RAID software in Windows?(The drivers were supported for 64-bit, but the software RAID utility wasn't!) Or tried to install my scanner? Needless to say in less than 60 days I had switched to 32-bit and I will be MUCH more thorough next time. Before you make the jump to 64-bit make sure ALL(every single itty bitty) program you use works fine on 64-bit OS before you jump to it.