There’s an excellent blog post up at Second Life that will help you to gauge your performance by graphics card:
This is long overdue, and I am sure will help people to figure out why they are (or are not) getting the performance they are after in SL. There are tips at the bottom of the post for improving your performance via preference settings. Ironically, we were just troubleshooting a performance issue at one of my high school labs yesterday. This will help. Hats off to Linden for providing this useful resource.