The 7600GS is much more powerful then the 9600, I'm puzzled by your loss of fps normally. It's a newer generation so you can't really compare processing speed with any graphics cards, they're funny like that with their pipes and 256bit buses etc.
Your cpu is holding you back now, that's why you don't notice any fps drop. The card can easily handle the extra AA and AF since it's not the one maxxed out. I always use the latest drivers because they fix some problems usually, unless they bring problems then I use older drivers. Can't hurt to try the newer drivers and see how it goes.