Can you elaborate on the SSD fix? I just ordered a 2nd PC (found a cheap quad core rig on ebay) that looks about like your setup, minus the SSD of course.

Theocrafting warning here -

I have to think that if adding the SSD helped that much, it must have been because something was getting loaded from disk in the middle of the fight. I thought for BG's (and all instanced content) that WoW would have loaded all it needed into RAM. I wonder if something was getting loaded into virtual memory (i.e. back to the hard drive) because you were actually maxing RAM usage. If you were getting lots of lag and bad FPS at the start of the instance, then I'd attibute it to stuff still loading from the HD. Maybe I'm underestimating how much stuff WoW has to load on the fly?