Everything looks good except for the video card(s)

A single 9800 GT will be better and more trouble free then two 9400 GT in SLI configuration.

Take a look at this link:

9800GT vs 9400GT

Now take a look at the "At a Glance" section and pay attention to the Memory Bandwith, FLOPS, Pixel and Texture fill rates. You will notice that if you double up the numbers from the 9400 they are still lower then the 9800.

Then take a look at the Details. Here you want to look at Shader Processors and Texture Units. Again, if you double up the number for the 9400 it is still lower then the 9800.

Add to that that SLI configurations and WoW multibox don't mix to well, they may work, but probably more trouble then what is worth.