I forgot to mention: both cards are OK for small resolutions with eye candy on medium or low. Good for HTPC. If your friend is a gamer, he shouldn't look at either of them. He should look at HD 4850 or even HD 4870. They cost more but they're a lot better. For example a HD 4850 has 800 stream processors, compared to 320 in the HD 4670. That gives it 2.5 times more raw power if I got my math right.
I would say a 9600gt is a gould mix of power and price i have one on one of my lower rigs and it smokes 1440:900 in all most games like cod4 maxed out ut3 maxed out grid maxed out!you get my point nice card nice price!
I have no idea what shader 5 is. But i can tell you that each card is about the same performance at both of those resolutions. If you are into overclocking though, the 9600 GSO will give you better performance in every game.
I agree with customisbetter. Strictly for gaming, and especially with overclocking, and if the cards are at the same price, I'd pick the 9600GSO. If the HD 4670 is $10 less or cheaper I'd pick the HD 4670. They'll be fine at 1280x1024.