JKay, there are hardcore gamers out there who use Vista and others who swear by XP. Older games will have more compatibility issues in Vista, where "older" is a relative term. Anything older than about 2 years is going to be less likely to run on Vista without either a patch from the developer or the use of Compatibility mode. Even then, there are probably some that just won't play in Vista. Some gamers prefer XP because of these compatibility issues.
Those who do play games in Vista find that the 64-bit versions are even less compatible with games than 32-bit Vista. Aero is not that much of a resource hog when you have 4 GB installed, but it's quite easy to disable it before a game and re-enable after the game. With a game launcher app, you can have this done automatically. The idea that games perform better in XP than Vista is outdated as updated drivers for Vista have basically eliminated the performance gap. Ed Bott explains this in his blog posting: