Compute Units/Ray Accelerators
Stream Processors
Memory Speed
Mhz

6600 XT is much better than 6600

You need to do research. I would advise you to speak to someone on building a pc or buy a prebuilt. There is so much to building a pc and replacing parts. I see your posts and you need more info. Not to be rude but alot can go wrong and you can burn money fast. I once blew 2 mobos and a psu because of wrong voltage trying to overclock... aha that wasn't cheap.