The RAM shortfall might be because your mobo is mapping PCI devices to the same address space as one of your live sticks of RAM rather than the empty slot. You could probably fix it by replacing your 4 1 GB sticks with 2 2 GB sticks. You might also find something in the BIOS.
http://support.microsoft.com/kb/929605

I've had issues with running 5x copies of WoW over multiple GPUs as well, although I'm now doing pretty well as long as I stick to 2 monitors.
Adding 3rd video card = *worse* performance?

Some things that helped me:
Reducing latency
[Other] PowerShell script to create symbolically linked (mklink) copies of WoW
Performance Issue