Can't really think of much else.
Memory issues would simply trigger a BSOD without fucking around with frame-rate drops.
You excluded addons or a faulty WoW installation.
You reinstalled the OS
You reinstalled the drivers
HDD errors would throw a BSOD at you
You have more than 2GB of memory so it won't be paging out to the HDD
You do have a power supply that is potentially underpowered for the system.
When you get a new PSU, make sure it is a proper brand. Not some crappy off-brand PoS like you have right now that only outputs 80% of the rated wattage.
Something like one of the following:
Seasonic X series, S12ii or M12ii
Corsair HX or AX series
Antec Earthwatts, Basiq Power Plus or High Power Gamer
XFX P1 series
Super Flower (might be hard to find in the States)
between 550 and 650W.