Overclocking 6420 Problems

Make sure your memory is at 1:1 (sync), you shouldn't have to increase any voltages though, perhaps try hitting 334fsb to see if you need to get out of the 1066 strap in order to get stable, it's a common problem for people to be unstable at the end of a strap (slightly below 266, 333, 400, and 500 for most users)

The strap is the fsb (intel's way of thinking) it runs at, and when you max out a strap, the board will change them for performance reasons I won't get into, but don't worry about what a strap is. Just try 334fsb. And running your ram at 600mhz is so I can tell where the bottleneck is at, not to mention, unless you have some crazy ram I highly doubt it will hit ddr2-1200 anyways (1:2 @ 300fsb)

yeah, my msn is in my profile

But if it's not stable, there's obviously a bottleneck, it's either voltage, cpu (highly unlikely), motherboard, or ram. If there wasn't any bottleneck then you'd be 100% stable right now