It could be that having all 4 memory slots populated is limiting your overclock - I've heard that before. Also a forum regular on here - CAT-THE-FIFTH - recently mentioned on another thread that nvidia 680i motherboards like yours apparently have issues overclocking Core 2 Quads. I've also read some reports that the 680i gets very hot, so the northbridge temperature might also be negatively affecting your overclock. How do I know all this? Because I'm currently putting an ASUS P5N32E-SLI under water in an attempt to clock my Q6600 up to ~ 3.6GHz
If I was you I'd remove the 2GB memory kit so you're running a nice stable 2 module dual channel system with 4GB RAM, and I'd see if I could get some cooling fans fixed in and pointed at the motherboard heatsink.
And one clarifying question: are you using the stock cooler that came with the processor, or do you have a decent after-market air cooler?