this is so cool, I actually have relative information to this. I own 3 computers, one has a p3 @ 800mhz, one has an amd 1800+ (1.5ghz), and the other has an amd 2200+ (1.8ghz). I also have a nvidia gforce 4 ti4600 and an ati 9200se. I did a little bit of mixing and matching and came up with the following results in 3dMark03...
on my 1800+ gave a cpu score of 315 and a 3d mark score of 1503
on my 2200+ gave a cpu score of 336 and a 3d mark score of 1561
so theres a small difference in score, not too bad. however i got a much lower score with the 9200se (as expected) even on the 1800+ (i haven't tested it on the 2200+... doh! sorry).
on my 1800+ gave a cpu score of 80 and a 3d mark score of 631
on my p3 gave a cpu score of 76 and a 3d mark score of 514
so there is a big difference in cpu score with a better graphics card. sorry this is rather an incomplete "sience experiment" being that i didnt test the p3 on the ti4600 or the 2200+ on the 9200se. i have a full report of all these tests saved in excel but i dont know of any way to link it to this post, sorry again.
hope that helps...