LOL, I still have a pair of Q6600's running. One has a GTX260 in it and can game most games at 1080p reduced settings. (The pcs are used as media servers and run BOINC)
You are probably limited only by the power supply in the PC and how much risk you want to take on ebay for the used card.
The Q6600 was a very good OC'ing chip, since this is a 'for fun' project PC consider turning up the clocks. Stock was 2.4 Ghz, Most people could hit 3 ghz no problem.