Fertilizer for SETI PC FARM

I purchased ten HP Small Form Factor DC 7800s from EBAY for $109 each including shipping. These come with intel Core 2 Duo E8300 processors at 2.83Ghz. These are producing 1500 - 2000 average credit per day. But I am considering whether to get more of these machines, or to upgrade the ones I have. I am thinking of low power GPU for math co-processing.

These computers have 250 watt power supplies.
I used a watt meter and found they pull no more then 70 watts during start up and full running.

Would such a coprocessor be a productive addition, or should I just keep on buying more computers

Thanks for any and all comments, observations, or recommendations!
David

I suspect that you will get more RAC per dollar spent by adding nVidia CUDA cards instead of more machines.

Although you may also have to upgrade the power supplies....

drop in some cheap quad cores..