Q: I understand both are used in the process of rendering, but which is used the most. I have a 8800GT and a AMDx2 3800, I'm debating whether I should upgrade my CPU for better rendering times, or should I wait.
A: The real time rendering is done with the video card (an 8400GS would usually be fine), but high quality rendering is done entirely by the CPU. GPU acceleration is the future, but currently they just don't have the computational precision and the software support isn't there yet. Definitely upgrade the CPU.