Thanks for the help guys, I made a few changes and now have a pretty stable machine - it's been running for a couple of hours last night and a couple this morning with no crashes so looks to be good!

Increased RAM voltage to 2.2v (was at 1.8v)
Increased Northbridge voltage by 0.2v to 1.5v
Manually set the latency settings
Increased FSB from 266 to 300MHz (which increases CPU speed from 2.4GHz to 2.7GHz)
Manually set the RAM multiplier to 3.0 so it's running at 900MHz rather than 1066.

I'll be keeping an eye on the temps to make sure these changes don't overheat it - is there any way I can check the Northbridge temp specifically?