Cheapest way to build an OpenCL render farm?

I'm thinking about getting something on the order 20-40 high end AMD parts for an OpenCL render farm.

If I could have it my way, I would buy just one CPU and maybe 4 GB of memory total. My application literally doesn't need anything more than that. The program is going to execute entirely on the GPU(s).

I want to know what I am looking at here. I'm not interested in crossfire. What is the best solution? Is there a way for me to avoid buying 10 cases?

What GPU should I target to maximize OpenCL performance per dollar? Another consideration is that if I have to buy 10 cases, 10 motherboards, etc etc. to support a certain amount of total compute power, that the overhead costs will shift the optimal GPU to something more powerful, so I can have less total graphics cards and less redundant CPU's, mobo's, etc.


So - I am guessing at the moment that I want something like an AMD 5830. Also- memory bandwidth isn't important to me, I just care about cores and the core clock speed and dollars. I don't think the 6800 cores are significantly better than the 5800 cores, but I'm interested if you think they are IF you have a source.

 

Get some of:
ASUS P6T6 WS Revolution supports up to 6 graphics cards - www.nordichardware.com

Probably a good idea to have a fast CPU with 8gb+ Ram to manege the results.