There would be no benefit to have them in a cluster instead of just mining separately. And unless your electricity is free a single ATI card will give you much more hashing power at a much lower electricity cost.
To be clear, if you mine separately on each of them, you're most likely duplicating a lot of effort and essentially only getting any unique work done on the system with the fastest CPU. If you use a mining pool, they handle coordination to ensure that work isn't being duplicated by members of the mining pool. I believe you could get the same benefit by using a dedicated CPU mining program on each system and pointing them to your own Bitcoin client as the server.
You can get more information here:
https://en.bitcoin.it/wiki/Cpu_Miner