With help from this forum I finally set up 4 render cows, plus installed a cow on the main machine for my limit of 5. All are 7.2 build, all are at least dual core and 2 to 4 gig of ram each. I'm on a private gigabit network, all pcs connecting at gigabyte speed. Vista 64, 64, 64, vista 32 and xp 32  in total.
I did a high resolution 30 second animation with 4 cows plus the cow running on the content creator and SNOOZE SNOOZE!!!
They spend so much time communicating, the rendering of each frame is only about 25% of the total time. With all five machines going the video took  24 hours to render whereas the solo quad core content creator rendered the same video in 30 hours. It seems like I just wasted time and money setting up this farm.
Now I also noted that the quad core vista 64 machine that runs all 4 cores at 100% in rendering alone only runs the cores at 25% when acting as a cow.
There is no info in the manual about optimizing (or cutting down on) overhead. Is this just the way it is?

One upset Farmer John