The video card may indeed be the issue here since you are using a 32 bit OS. If the lost memory is pushing you below 3GB (it should be) physical ram, then running 5 clients on the machine would cause it to start paging memory which is where you take a huge hit on performance.

I'd be curious to see how it performed with a 512GB card, or with Vista 64 on it instead.

Other than that, the best advice has already been given...

/console maxfps 30
/console maxfpsbk 15 (I chose 15 because @ 12 I'd sometimes lose /follow, but @ 15 I never do, and it's background, you don't need 30 FPS)

Setting the secondary clients to minimal graphics settings helps, turning off sound on them helps a LOT.

Another big tip, turn off the "Play sound while in background" (This may be off by default) as it disables the sound engine for 4 clients out of the 5, which makes it a lot less noisy as well as saving big CPU hits.

Another thing, since it's a quad core you have to set your affinity or WoW will only use CPU 0 & 1.

PowerShell script to create symbolically linked (mklink) copies of WoW

That thread explains how to create simlinked copies of wow, this has helped me in both performance and sanity, as you can have different settings but still access a single set of data files. You patch once, and all clients are patched. Update 1 set of addons, all are updated, etc.