The PC 6600 VGA has the nVIDIA GeForce 6600 GPU with a core clock of 300 MHz. It will also use 128 MB of DDR memory with a memory clock of 500 MHz. The PC 6600GT card will use the GeForce 6600GT GPU with a core clock of up to 500 MHz. The memory clock for the PC 6600GT card is 1 GHz.
Both are, of course, fully DirectX 9.0 compliant, and both will support Shader Model 3.0, a technology very important to games like Doom 3 and Far Cry.