I found a 2gig 6950 is about optimal before performance starts to take a hit............. you will find games that will be more cpu bound than gpu bound with this set up. (so a *2gig* 6950 or 7850 ) the 660 will be severely cpu bound. ( overkill ).,.... 560's suck.
last retired machine was a q9550/8gig ddr2-800/I ran multiple cards on that set up at 1920x1200.