About a week ago, Valve began a new survey for users of their Steam service and then they've kindly released all the interesting information they got on a constantly-updating results page open to the public.
At the time of writing this article, the survey shows that only 21,467 (5.41%) gamers use Windows Vista to play on steam compared to 368,779 (92.89%) gamers using Windows XP. About 55% percent of surveyed gamers use Nvidia's graphic cards compared to 35% using ATI's, but expect ATI's share to increase due to the recently-announced Radeon HD â€" HalfLife 2 bundle.
On the lighter side, only 1.32% have multi-GPUs, with only 2 users having a 3 GPU-SLI system.
Don't forget, this is just a snapshot of the numbers which change constantly. You can check the full list of up to date statistics by following the link available in the downloads tab.