CPU/GPU CUDA Acceleration on Mac Pro 1,1

[Neox]

Hi,
I have aMac Pro 1,1 (dual Xeon 2,66 4 Cores in total) with Radeon X1900XT, running DVD fab mainly for BluRay BD50 to BD25 shrinking.
Booting via bootcamp with Win7 ultimate.

I am currently evaluating if changing the Graphic Card to a Nvidia 8800GT Mac (I am afraid newer ones are not supported by my Mac).

Will in this case DVDFab ripping time significantly improve or not?

My thoughs are:
- CUDA should be supported (please confirm).

- Also, what it is not 100% clear to me is: will all processing be handled by GPU (therefore, is that card fastwer than my 4 cores?) or is the GPU processing power ON TOP of the 4 CPU cores (hence ripping speed improves) ?

Thanks in advance.

[Neox]

Clear AGJ, thanks for the answer.

Let's assume I find the way to have CUDA under BootCAmp (I am actually still trying to figure this out on the blogs...): Will the new hardware configuration improve re-encoding speed?

Forget for a while I am on a Mac.
In general terms, is the GPU something on top of the CPU or something that replaces CPU, hence to check for speed improvement it's still necessary to compare CPU vs GPU?

Thanks