You'd be really stretching it CPU-wise to run 5 on your PC. You can run 5 with 4GB of RAM, assuming you're running 64-bit Windows, and you can run 5 with an 8600GT, but you're going to be CPU-bound with a 2.66GHz dual core, even if you overclock it.

What you'll need to do is have the non-main windows running at as small of a resolution as possible. WoW will automatically adjust its resolution to match the size of the window, so you can just click and drag to resize the window. This will immediately improve your FPS, and if you're limiting background FPS with maxfpsbk to a number lower than they will actually run at, it will free up CPU and GPU time for everything else. Additionally, make sure your Multisampling setting in the WoW Video options panel, under Resolution, is set to a 1x multisampling option (e.g. 24-bit, 24-bit, 1x). Any higher and you're making your PC do a lot of extra work that's not going to help you multibox.