I have almost the exact same system (See my sig), just no overclock, and a smaller resolution monitor.
I'm currently planning a ~$750 replacement system, working on a Sandy Bridge 2500K or a Thuban 1090t. I'm waiting to see how the 2500K does.
A recent tom's article of 20 popular games suggests an average of 2.5 cores in use, so an update to a quad is highly recommended. Several recent games run best on at least 4 cores: Final Fantasy XIV, GTA 4, Metro 2033, Civ V, Starcraft II, etc.
for $500, the best option will probably be to get something like a GTX 570 or 580