GeForce 8800 GTS 512MB is a high-end GFX based on the 65nm variant of the first-shader unified architecture.
It's based on the G92 Core and offers 128 Shader Processing Units, 64 TMUs and 16 ROPs, on a 256-bit memory interface of GDDR3. The central unit runs at 650MHz and the memory clock operates 800MHz. It will consume no more than 135 Watt.
It's therefore related to GeForce 8800 GTX (in terms of Core Configuration, not technology) and should perform just a little worse at lower resolutions and considerably slower on higher resolutions. It's significantly faster than the 640MB variant based on the G80 Core.
Today's modern demanding games can be played at medium (some) or high settings (a good deal of them) smoothly. However, DirectX 11 based games aren't supported.