Possibly Vista's biggest selling point to the PC Gaming market is the fact that Vista is the only way for users to get DirectX 10, and to play games in their beautiful DX10 glory. Its abundantly clear that games such as Crysis and many Next-Gen games coming out will benefit from the DX 10 upgrade. However, current statistics show that this graphical upgrade hasn't been enough to shift the PC Gamers towards Vista.
Currently there is no way to play DirectX 10 on Windows XP, and the only way to get the best possible gaming experience would be to upgrade to Vista If you are unfamiliar with the differences between DX9 and DX10, see here for a video comparing the two. However, this difference hasn't been a big enough to push many gamers into upgrading to Vista. The statistics from Valve show that less than 15% of users are using Windows Vista, with over 80% of users running Windows XP.
Another poll shows the exact same information, that many users don't see the need to upgrade to Vista right now. Perhaps once a truly great game comes out that is compatible with DX10, we will see the market shift towards Vista.