Right - update time. I think i have cracked it. I swapped the old corsair stuff out for some g.skill memory, and, as it turns out, am back to using the corsair.
Im pretty sure that instability issues in vista itself were caused by slightly undervolted RAM. I had heard that the board overvolts like a madman, and so wanted to be careful with the RAM voltage, but i guess that it was not enough. Perhaps this (fairly) widespread information needs to be looked at again. I upped the voltage a few notches on saturday morning and since have run F@H for 24hrs+. Whilst im sure this probably is not as good a test as prime, it achieves more, and to be honest the fact that this was not working in the first place was the cuase of the problem.
As for the memtest issues, this one is a little strange. I spent quite a while on google and in the end found some people using the p5q motherboards also having some issues with memtest. I cannot remeber where i found the information (on some godforsaken forum somewhere else!), but it was suggested that disabling USB legacy support/removing any USB hubs at startup would provide a solution, and that the fault was to do with the BIOSes. I did this tonight - unplugged a USB hub/keyboard and disabled USB legacy support in the BIOS, and ran memtest 2.01 from a bootable cd. The test ran for ~2.5 hours, completed three and a half passes, and did not find a single error. Since the memory could barely make it through two passes before (and the g.skill failed almost immediately) i am inclined to think that this is enough of a run to prove that the problem is a BIOS/USB issue (i'll let F@H be the judge of any long term instability). So, thats my fix, if anyone is interested. I'll see if there are any long term issues, and im keeping the G.Skill RAM in case something does go wrong, but for now i think its back to folding!
@BlackAle - this is something that may be fixed by Ket's BIOSes (im sure they are fantastic - he has clearly put in a lot of effort and i appreciate the support he has given the community) - however, since the issue seems to be memtest exclusive for me (for now), im going to stick to the official BIOSes released by ASUS.