Well, I personally would say go with XP if you have it, but go with Vista if its going to be a new build.
Vista is going to continue to mature, sp1 is available now via torrents and will officially be available in a few weeks.
I am currently running vista with sp1, it works well. I used it for about 8 months, ditched it for xp for a few weeks.
In all honesty I missed the quick find on the start menu, type in the name of the program and it pops up. I also missed the sidebar gadgets. I tried looking for an alternative, but no one makes those same ones.
As long as you are up to date on drivers, download the latest program versions and updates. You should be ok.
I game on my vista machine regularly, Crysis, Cod4, sins of a solar empire, guitar hero 3 etc. Even supreme commander.
I have had no issues really. But once again you have got to keep up with the drivers, video being the most important.