Wolvan I agree, try a different slot for the card. Try booting up and see what happens, then go back to the other slot and try again.
Are you overclocked? Not the vid card (you said no to that above) but other things? If so might wanna tweak things back.
Also should check your voltages, make sure they're not messed up -- could use BIOS but often that's not accurate, so if you can get into windows check it there.
And if you have a voltimeter you can check the power supply to verify that's not part of the problem, though I don't think it is.
Oh also -- you mentioned you upgraded a few weeks ago, what did you add to the system?
MMO's Played Include The Realm, Meridian 59, UO, AO, AC2, DAOC, WOW
DAOC 2001-2006, Toons Included:
Kestrel (sorceror, alb perc) --> Hasil (infiltrator, alb perc) --> Bardif (healer, mid guin) --> Chianan (bard, hib merlin)
Sacred Brotherhood, Ascendency, Solarius
WAR --> Hasil the Witch Elf
WAR --> Keztrel the Zealot