There are too many variables, and your question isn't properly defined.
Which is faster, is the Core 2 Duo, the video card won't make a bit of difference in that. You need to add some sort of condition to gaming or rendering if the GPU is factoring inot it, then you should also note the app you are using as some will utilize the 2 cores and some it won't make a bit of differnce to.
A Turion isn't Sli just because it is a Turion, the system would need to be Sli. So if we are talking baout 1 video card vs. another, the 7900 GS is probably a faster/better card.
A Core 2 Duo would wipe the floor with a Turion CPU vs. CPU, that you are right.
You may want to go to my page and check out the Mobile GPU comparison page and the COre Duo & Turion page, it is in the menu under SPECIFICATIONShttp://www.killernotebooks.com/images/logos/glass_killer_small.jpg