I have a Q6600 paired with an Asus P5N-T Deluxe motherboard. Overclocking even 250mhz on this board frequently causes a random lockup ingame, requiring me to restart, or it simply fails POST on boot. Due to the fact that I can't really overclock it, I'm pretty sure it's affecting the GPU performance significantly. I have a GTX 470 - and in quite a few games, I'll have fairly consistent FPS (35~50) but there's moments where it just randomly drops incredibly low (12 and under). I also have a noticeable amount of stuttering at high resolutions. This problem is especially prominent in Dead Rising 2 and Fallout: New Vegas. Any suggestions for a new processor that'll handle new games?
Without OCing, you're using 2 or 3 2.4GHz cores to game with. My own 4-yr old QX6700 OC'd to 3.2GHz with an HD 5870 is outperformed by the i5 750 (3.0Ghz+Turbo) + HD5750 alongside it, gaming at 1920 in WoW.
Frankly, if I were toying with replacing the processor I'd also replace the mobo and go for the i5 750. Combining that with your 470 will produce quite a nice gaming system.
If your psu is up to things, the cpu/mobo/4GBmemory would run $380 minimum, and $430 with a more featured (ie, SLI/xfire) mobo.
OTOH, the next 2-3 months should give us the Sandy Bridge cpu which should be even better, or drive down the cost of socket 1156 a bit more.