I wanna contribute my experience to this as well...
My PC ain't that great but it's ok... C2D 1.8ghz and 2mb ram

anyways, i've experienced in a couple of times where my fps dropped to 20 - 40ish...
I tried restarting WoW but that didn't work... after about 10 mins, fps went back to normal.
I alt+tab only to find out because my antivirus was running its schedule scan :S