Your computer reminds me of my Frankenstein machine: 2 underpowered video cards, an array of old hard drives but a strong mobo/cpu/ram Your system has a few bottlenecks, namely video and that USB hard drive, but its a start.

Video cards: WoW does not play well with multiple GPUs. Only with the use of specific setups (I believe IS can handle this) can WoW be launched on specific/second GPUs. So if I had to guess, your second video card is doing nothing more than hosting the dvi jack that your monitor is plugged into.

Video Cards (continued) You say your 8800gt is a 640mb G92 model??? I have never heard of this. I may be mistaken, but iirc, this card has the gpu prior to the G92. Buy a new 200 series, you will not be disappointed (especially with a 285)

USB hard drive? Really? Dont do that! There are dozens of threads full of anecdotal proof that symlinking the wow\data folder to multple copies of one installation is the way to go. Your multiple full installs accross 4 HDDs is just plain scary, you are wasting GB upon GB and not gaining much performance. But the fact that one of these drives is trying to shove its data through a USB cable makes me wonder if that instance loads slower than the others.

CPU: My Q6600 has been stable at 3.2 on air for two years, Ive heard claims of higher. OC THAT SHIT. If you've got an extra $220 burning a hole in your pocket, the Q9550 would be a hell of an upgrade.

Core Affinity: As stated above, WoW does have limited multicore utilization built in, but this is limited to running sound on the second (#1) core. Assuming you have an audio card, this is unnecessary. If not, allowing your main instance (or the one that runs with sound on) access to a second core, would be beneficial. Your affinity should be ok.