I have to side with Vimp. He is right overall and specifically because some programs are, indeed, more dependent on the CPU while others are more dependent in the GPU.
Scottchen is also right however. Putting a sate of the art video card into a lower mid-range machine is a waste in the sense that it is like installing heated leather seats in a 1988 Chevy with 200K miles on it.
Strive for a harmonious system where all components are the same generation. If you manage to build such a system it will bring you a lot of joy over a very long time (at the very least 2 years).
You will get better performance from your PC with the nV6800 but it will not be as noticeable and the actual boost may not feel like it was worth all the money spent.
Finally, if you do upgrade your CPU and motherboard, say a year from now the video card most likely will become obsolete and the cycle will repeat only now you will have a kick-ass CPU and a sub-par VGA card. Try to avoid this because this way your performance will always be lacking.
The hottest CPU of today, the Pentium 4 Extreme Edition, is barely adequate for the 6800 but they work together well. Go for it, tiger.
<font color=green>Stingy people end up paying double. One kick-ass rig that will go strong for three years or one half-decent one every year?</font color=green>