Ugh! That video card is by no means a "mid end" card. The 8600's are craptastic for performance.

Get a better card and no you dont have to go ballistic with it. An 8800 series will blow that 8600 out of the water.

Then since your on vista I would upgrade the ram.

Your processor is fine and with a better vid card and a lil more ram you should be able to run all 5 wow's from one PC. Definitely the 3 you want to run at this point.

I have a very similar build to yours the only difference is I have an 8800gts card ($129iirc) and winxp with 3gigs ram. Otherwise my processor is same as yours AMD64X2 2.1ghz and I have run 3 instances with full graphics and averaged around 50fps on all three.