You should try running it all off your primary PC. I'm currently 5-boxing on an overclocked i7 950 (@3.6Ghz) with 12GB of system memory. I got two 24" screens hooked to that box and have one client fullscreen and the other 4 on the second screen. 50% downsampled 1920x1200 is close enough to the resolution of your tertiary screens (1280x1024) I think you wont lose much here. I'm running all clients in DX11 with one client on high settings and the others on low-medium, the slaves capped at 20fps.

For the GPU you most likely want a non-SLI setup because SLI will give you only very low increase in performance but alot of problems and issues. My GTX 580 is already overkill for that setup since I'm running into CPU-limitations when LFR-ing rather that GPU-limits. I'd still replace that HD 6870 because WoW performs better on Nvidia cards and you want a card with 3-4GB of video memory for 5 clients.

Performance-wise its quiet good on a single box, fps on the main client is 25-40 in capitals. In 40-man BGs it dropped down to 20 in crowded areas but Blizzard solved that problem by removing /follow from BGs. In 25-man raids (LFR) it gets worse by dropping to 10-15 which is close to being unplayable (yet I still do good dps I just dont move). Problem here is the CPU (I see all cores close to 100%) not the GPU.

As for software - ISboxer supports multi-computer configurations I just cant comment anything about that because I never tried that (running of one box only). I think its recommended somewhere to run all clients at the same resolution when using ISboxer.