The latest update of Valve's Steam Survey indicates only 9.60% of all Steam users have support for DirectX 10 games:
A couple of other interesting observations:
2GB memory is the most popular memory configuration, 38.25% of the Steam gamers have 2GB of RAM in their rig.
58.50% have a processor from Intel, 41.5% one from AMD.
Single-core processors win with 59.06%, followed by dual-core CPUs at 36.61% and quad-core processors at 4.31%.
Gamers appear to prefer NVIDIA, the top five video cards come from NVIDIA. The GeForce 8800 series is the most popular video card series with 9.37%, followed by the GeForce 7600 at 5.72% and the GeForce 8600 at 5.34%. The three most popular graphics cards from ATI are the Radeon 9600 (3.10%), Radeon 9200 (2.57%) and Radeon X1950 (2.36%). The ATI Radeon HD 3850, 3870 and NVIDIA's GeForce 9 series don't show up in the list because their marketshare is too small.
25.73% of all Steam gamers have a widescreen display.
Around 30% still uses a AGP graphics card
The most popular primary display resolution is 1280 x 960 (39.17%), followed by 1024 x 768 (32.02%) and 1680 x 1050 (9.12%).
3.49% of the Steam gamers have more than one display
Windows XP has a marketshare of 80.88%, Windows Vista 14.99%