Ok I've made a bit of progress with this board, sortof...
http://www.superficial.net/miscstuff/p5w64_450.jpg
Setting memory to 3:2 meant I could ClockGen up to 460 before having problems (I know screenshot shows 450, but I got higher after it). Setting it to 470 locked the machine.
This is with vDimm 2.1v, vMCH 1.7, everything else AUTO (except vCore but that's irrelevant I think?). Manual memory setting in BIOS, then everything other than the 4-4-4-12 changed to match SPD.
Does this mean my G.Skill memory is not capable of running over ~410FSB 1:1? If so, how come I can boot into Windows and run Prime when its running 500Mhz? (not on 1:1)