This is my first build and overclock, my aim is stable at 3.00 GHz which I'm pretty sure should be achievable. At the moment I'm not too sure about the System Memory Multiplier even after reading several guides. So I thought the best thing to do would be to post the settings I'm trying to use and see what you think. With these settings it won't POST and reverts to the old settings after two unsuccessful boots. Anyway, here are my specs:
CPU: Q6600 G0 version
Motherboard: Gigabyte EP45-UD3R rev. 1.0 (updated to F9 BIOS)
Change System Memory Multiplier (SPD) to 2.00a or 2.40b. Also change CPU Vcore: 3.250. That should be just fine. If not slowly increase vcore until system is stable. If nothing works take out a couple sticks of ram. I have same cpu and mb and this worked for me. Best of luck.