64 bit is the way to go. An addition:
If you get a second HD (or partition your only HD) then do yourself a *huge* favor - manually install all games to a different location than %ProgramFiles% / %ProgramFilesX86% (which usually ends up expanding to C:\Program Files\ and C:\Program Files (x86)\. There is a reason for that.
Vista locks down security for protected folders - including the Program Files folders. You need special permissions to work in those folders, add files, make mods, etc. that can get to be a real PITA. By using a separate folder (or, as I do, a separate *drive*) you bypass the folder restrictions, giving you free reign on the folder(s).
As an example, I have dual 500 GB drives and a 250 GB Drive. my 250 is partitioned ~150/~100, showing as drives E: and F:. on E I have *all* of my 'special' folders - Downloads, Pictures, Desktop, etc.... - this makes for simple system re-builds. on F I have a single folder - Program Files. It keeps all of my games that I install and play and mod.
Most modern games won't really have much of an issue unless you start making .mods / creating maps / etc. Older games, however, that were made before Vista, can have some PITA experiences when it comes to playing them out of the permission locked Program Files on your normal install.
Another benefit is that I have set my AV to not actively scan F:\Program Files, thereby keeping my system from slowing down during gameplay - with the net result being that I can leave my protection on and running and it not affect my gameplay, and vice versa.
I am sure others here will have something to say about this, but this is my practical experience from when I started testing Vista almost 2 years ago, on my old dinky system (that was as bad, if not worse, than Bink's old system!) - a P4 2.0(A) GHz machine with 1 GB RAM and 3 7200 rpm IDE drives.
This has been my experience