6600GT is faster, x700pro is cheaper.
It's up to you man, $41 is a considerable percentage of the 6600GT price, go check some benchmarks and make the decision if the performance is worth it.
Ultimately you have to make the decision on whether the performance is worth it.
To the previous posters, well the 6800GT is faster in pretty much everything, I'd say it's worth the premium.... the 7800GTX is faster in pretty much everything, I'd say it's worth the premium....
Everyone has their price limit that they will pay for the performance. The statement that any performance imporvement is worth any price is a ridiculous statement indeed.