Here is my setup. I do steady 45 - 60 fps in dalaran with my main and 30 fps with my slaves because seems they get capped on 30 fps when "minimized".


MoBo = Assus Striker Extreme (comparable to yours)
8 Gb RAM ddr2 @ 800 (worse than you)
1 x Nvidia 8800 GTS (you have two comparable video cards, so you should do better)
Intel q9650 cpu default clock (yours is comparable to mine)

What you need to do is configure core affinity in wow. By default all your wow instances will use only the first two cores, so you will have 2 cores at 100% and other 2 cores at 0% usage. This alone will give you a significant boost.

Then buy more hard drives. I have one system HD with the main wow instance, then a secondary HD with two other wow instances and then a raid 5 array with other two wow instances. This will not speed your game much, but will reduce the load time when you hearth to dalaran or zone/instance loads.

The only upgrade you need is to buy more hard disks (one more will do the work, but two more will give you similar performance that playing a single toon (no multiboxing) on this same machine you have.
The CPU affinity will fix the frame rates completely.