You need a 6600 GT or better (6800, 7600, etc) to play WoW on a single machine with decent framerates. Look at the 8600s as an option, about $120 each, depending on the model. I do not suggest 6500s or 7500s or anything less than their mid tier models as the fps start to suffer greatly and you don't save much.

I hear you on the silent part. Moved all of my computers about 30' away and sound insulated the walls. I love it.