You are havng the exact same issue as me. I get around 75fps on my TNT2 card as well. I have been talking to MANY people that also have TNT2 cards and they are all experiencing the same very poor performance with the latest 5336 drivers.
The last set of drivers that actually worked normally were the 'leaked' 4620 drivers. Every release after that has given my TNT2 card grief. I have replicated this problem now on several machines at work with TNT2 cards. So it is definately a driver issue and not 'config' problems.
Time to fix this NVIDIA. Just about every computer here at my work uses a TNT2 chip so you can't dump driver support for these chips yet!!!