Hi there - thanks for your replies. Unfortunately, the x2 multiplier is the lowest multipler on the gigabyte - it takes into account the DDR2 multiplication. So 270 FSB is actually rated by the motherboard (and on cpu-id) as DDR2- 540 on a x2 multiplier. To demonstrate, I've currently got the multipler at x3.33 on 270 FSB which is giving me a ram speed of nearly 900 and it's comfortable at these speeds.
What I want to do is obviously increase the fsb (at which point I'll lower the multiplier). I just keep running into the problems listed above.