GeForce 8400 GS is an entry-level GFX based on the 80nm variant of the first-shader unified architecture.
It's based on the G86 Core and offers 16 Shader Processing Units, 8 TMUs and 4 ROPs, on a 64-bit memory interface of DDR2. The central unit runs at 450MHz and the memory clock operates at 400MHz. It will consume no more than 40 Watt.
It's therefore a GeForce 8500 GT, slightly lower power consumption and no SLI support.
This GPU is only suited for a 1024x768 (or less) resolution. Most of today's games will only run at low settings and extremely demanding games might not run at all. DirectX 11 based games aren't supported and its performance will decrease drastically, as the resolution increases.
Note: This GPU had two revisions which added HDMI support, DDR3 memory and different core configurations.