I've been running my Q6600 at 400x8, 1.328v idle, 1.312-1.328v load for some time now. Seems to work great (Prime95 for a couple of hours). However, I now have some free time that I thought I might use to improve that OC a little. The specs (bought at the end of last year) are as follows.

CPU: Q6600 G0 L726A951 (VID: 1.2875v)
CPU-Cooler: TRUE + Scythe S-flex 1600rpm @ 1300 rpm
Mobo: Gigabyte P35-DS4 rev. 1.1 BIOS: F12
Ram: Corsair TwinX XMS2 CL5 2x1GB + OCZ 2x2GB 800MHz (OCZ2N800SR4GK) @2.1v
Case: Antec P180 (first revision)
PSU: Corsair 620HX

Currently every voltage except vcore and vMem is standard. Is there any way to know if it's vcore that is stopping me or if it may be vFSB or vNB? Are there any guidlines I might have missed when it comes to the voltages other than vcore in regards to overclocking?

I know 4 memory sticks tax the NB/MCH more but it's been working at standard vNB without a hitch since I bought the extra ram.