Depends on the resolution you play at, and amount of eye candy you turn on. That being said, the 6600GT is about double the power of a 9600Pro. If you have a 9600SE, even more so.
1280x1024 with all effects including the happy fun heat distortion should be doable with a reasonable frame rate (20 min), even with pet classes hanging out.
You make the call if it's worth it