Again, thanks for all the advice, huge eye-opener. Oh, and a Merry Christmas to you all!
Last night I realised I've been doing the wrong test in Orthos to really stress the CPU on its own (d'oh). I was doing blend tests but after revising the sticky guide here, started trying Small FFTs and my CPU is far from stable under that :/. After about 3-7 secs of starting the test the responsivity of my whole system decreased massively, so I was hitting 'stop' with no response for about 10 seconds. This problem has affected both blend and FFTs tests now, even after reducing the FSB to 380.
The thing is, I've been playing games such as Farcry 2 and L4D for hours at a time without any problems (this was with the FSB at 410). I was tempted to just say screw the tests as this is what I use my computer for in the main, but I'm sure I'll see the effects of that in the future if I don't stabilize now.
I'll keep everything said in mind, especially the stuff about RAM timings and ratio thanks! I think I've been a bit too hasty, only doing 2 hour tests on settings I'm concerned about. Need to start again and do overnight tests.
Cheers, and hope Santa brought you more than a scarf and a pair of wooly socks ffs (I'm not bitter in the slightest!).