Try this. Have windows manage your wow clients on all 4 cores. Set the affinity to 15 for each client and then all 4 cores are engaged for all 5 clients. I did this and saw a large performance increase. In addition, on my machine and settings 5 wow clients take up ~6 GB of ram. A 64 bit OS helps too. I don't know what your display hardware is like but if you have 2 monitors and 2 cards you can look into InnerSpace for multiple GPU management which will also help with performance.

But first, check your ram usage under task manager. I think that may be the culprit. If you have enough then try running all clients on all 4 cores.

Drommon