If you need to buy an OS, I'd say go with Vista.
If you already own XP, then I'd stick with that for now.
Vista will give you DX10 support for games, but you're current video card will struggle using it.
Playing in DX9 will give much better framerates, and at this point in time, there's not a huge amount of difference in visuals, especially when you're actually playing the game.
Although 64bit computing isn't really a huge thing right now, Vista x64 will play games just as well as Vista 32bit, give or take a frame or two, and also give you the option of adding 4gb or more of ram and being able to use it all. (Subject to motherboard).
Vista 32bit is limited to 4gb or ram, and you wont have all that available to use, so no real reason to go with it unless you have 16bit games and apps you need to run.