It didnt work. Fitted the new (and very impressive looking) power supply, which was a Tagan 480W, and checked everything was connected ok.
Switched on the PC and....nothing. Out of 15-20 attempts it loaded up past the boot up screen only once and then crashed after a few minutes
Tried disconnecting a couple of drives to see if this made any difference but didnt. Also tried clearing the CMOS but no luck. Its very difficult to troubleshoot as cant get the thing booted up long enough to investigate!
I think I'll have tp try and borrow my mates Nvidia card again to get the system working again and if that works and my card works in his (and I cant find a solution) might have to reinstall XP which fills me with dread as got so much installed would take ages to back up and rebuild.