Aside from GT4 specifically being a resource hog and not likely to run well on your computer with either video card (or the CPU) I would think either will pair well with your processor.
A 9600GT will run better, but will require more power and a 6pin connector. A 4670 is a great card, and is power exclusively off the card slot. Both cards will perform well at 1440x900, though a drop to 1280x720 in gameplay may yield more consistently strong performance across multiple titles.
He said he has a choice between the two and doesn't mention the source. For all we know he has a friend with both cards willing to sell one of them to him for cheap. I agree that a 5670 would be a better choice than both, but it does cost more than either of those cards.