i would hold on the purchase, going for a Dual CPU solution is incredibly risky if you don't know what you are doing. so far you've been presented with 3 things you didn't know about and should you have bought the hardware you would end up with it sitting there looking pretty. also, being restricted to Server CPUs means changing the ram for Server grade... not good at all. in the end whatever extra cores you gain, will result in a negligible performance increase due to the slow RAM, and the weaker cores.

a better solution and much easier to set up would be to invest on a second PC (i7 at most), and link them using the multi PC functionality that IsBoxer has.

keep the main power rig for your main group, then the second PC running the filler characters.

and btw, im assuming you are running at least 20 clients. anything under that and you are just throwing money away, and any performance problem is more likely due to poor configuration on your side rather than some hardware bottleneck.

i would look at those titanXs, SLI setups run like ass with multiple instances of the same program.