OP, our situations are VERY similar. Our boards, aren't the newst, and we have C2Duos OC'D. After much researching and threads it has come down to these cards from weakest to strongest, the last 2 being VERY close.
gtx 460 hawk if you want to spend 135~
gtx 560 Ti for 180~
radeon 6950 for 200~
All single cards, forget SLI or Crossfire. We game on single monitors that aren't even that big.
Also a lot of games these days aren't even quad core aware yet, most if not all are duo core aware, as Ayeka pointed out before. I'd say it's not time to scrap your rig yet!
Like I have already stated the upgrade I did from dualcore to quadcore resulted in a 300-400% FPS increase in all the games I play. Not just the CPU hogs like BBC2 and BO.