Yes. Vista is running pretty much any game and running it fine. It's come a long way since it's release. Patches, new video drivers etc. There may be a few exceptions here and there. If he has any games he simply MUST run then tell us which ones and we can check on it.
If you are getting a new PC with 2 gig or more of RAM and a dual core modern processor then Vista is a very good choice for an OS. XP is perhaps slightly faster and more compatible, to a degree, but in exchange with Vista you get dx 10 and the future.
Plenty of us hard core gamers here are using Vista and liking it just fine. Vista 64 and 4 gigs of RAM is a very popular choice.
I tried Various DX10 games and I couldn't tell the difference from the DX9 rendering. That's not to say it will not get better, but if you want to play DX10 games you'll need good hardware. Many cards claim Dx10 compatibility, but don't have enough power to produce playable FPS. Don't go lower than a 2900/3850 or 8800GTS for DX10.
The framerate gap in DX9 has closed according to the benchmarks, but be warned some older titles will not run on Vista. Some games will have fixes, but it's hit and miss.
I personally prefer XP, it maybe old but it's faster than Vista in most things. I'd rather have somethings that's older, but more stable and less buggy. XP will also make use of Quads and Dual core.
I have been trying to play bf2142 on vista ultimate for the past week and I have been having a lot of problems. The update won't install unless I try numerous times in safe mode, the game crashes randomly, and if I manage to get a game to load in multiplayer it closes instantly because of some modified files garbage.
If done a lot of searching on google, and it seems these problems are common, but there isn't a definite fix available. I think some people are lucky enough to get it to work, but from my experience I wouldn't count on it.
I have Vista 64 and 4GB and playing a lot of BF2142.
The game is working great for me. I must run it as "Administrator" (PunkBuster). When crashing, the server that I was on did crash most of the time (can not rejoin it) or I believe my overclocking is not 100% stable.