Yeah, except for almost everything, Vista isn't that bad. I mean, even Microsoft themselves advises not to buy Vista.
For games, I think the most relevant thing it to figure whether you want/need to play DirectX 10/11 games. Officially this is only supported by Vista and 7, but using an unofficial patch it will also work on XP. I don't know how reliable this patch is, and if it works without problems.
Almost every modern day game still work perfectly with only DirectX 9; in most cases, the difference is negligible.
So, I would recommend Windows XP. It's old, but that's an advantage, because it requires much less resources, so it has better performance. I too game using XP, and it works great.
If you really need DirectX 10/11, and you don't want to try the unofficial patch, I would recommend Windows 7. It's better than Vista (although it isn't so hard to be better than Vista), and has good DirectX support.
But, if you have enough time, wait until they release Service Pack 1 for Windows 7. Just to be sure.
- Black
GENERATION 9: The first time you see this, copy it into your sig on any forum and add 1 to the generation. Social experiment.