Hey, I asked if you were around in IRC and you were idle, so I'll reply here :_D

IMO If you'd be better off going with 2 boxes if they can support the 2 wows decently. Tune down the fps, its not that crucial (to me anyway) how many fps my clones are pushing so long as they don't lag out. If they can't support 2 wows easily, go the 4 box route. I use 4 clients, 2 of which are sub 2.0 ghz machines with ~64mb AGP cards. I don't have any problems with each running a single session. 4 of those Dells should do you fine if you wanted to go the 4 machine way. The thing I like about 4 machines is you don't have a lot of loading lag waiting for each instance to load up.