What is a Phantom processor? Did you mean Phenom?

As for PCI Express x8 or x16, they mean absolutely nothing unless you're trying to break world record
benchmark numbers. The difference between the two is negligible.

WoW is CPU dependent, so if you're already at 100% processor usage I doubt a video card will make a big
difference unless your on-board GPU is sucking up a lot of CPU as well (I have no idea). Any video card you
can find for close to $100 I guess will work but I feel like it's putting a band-aid over a gaping wound. An
nVidia 430/440/450 or ATi equivalent card is pushing your power supply, though.