I'm running this game just fine on a GTX 660, which is just slightly better. I do experience some minor FPS lag in the new 24-man, but nothing huge. I'm in the weird camp where I don't update my graphics drivers if I don't need to. My current driver version is 21.21.13.7254. Perhaps you could try rolling back to these drivers? I do experience minor lighting bugs in some buildings in the game, but again, nothing too bad. Not sure if that's driver-related or not. You could also always try running a more aggressive fan profile (100% under load) if you're not already. And as a more simple suggestion, try reseating your graphics card, maybe blowing some dust off in the process. Sometimes mine gets knocked loose a little and causes crashes.