CPU or GPU in clusters ?

I am puting together a small render farm to run by Compressor 4 / Qmaster. Do we know if The graphic cards in the cluster machines are effective at all in rendering - or it's the CPU that does the work ? The reason for asking is; some powerful intel Xserves could be bought for $500 these days. But, the graphic cards in Xserves suck wind ( they are of lower end ). If the graphic cards contribute in cluster rendering, then newer MacMini servers with higher end graphics cards make better cluster machines.

So, are Graphic cards in " cluster machines " effective in rendering ?