1. What are you doing with these machines?
If they are for precision computing (e.g. physics simulations or financial analysis), or they are for mission critical computing, then we can take consumer CPUs off the table--you will want ECC support.
2. How many of these are you building?
3. Will they be run from the same place?
4. How much uptime will they have?
I ask these because as you cram more machines into the same space, the cost of the storage space, cooling solutions, and power to accommodate cooling solutions become non-trivial. If you have a power hungry CPU that's cheap and a power efficient CPU that's expensive, and they offer the same computing power, the expensive CPU can easily be more economical, because:
a. You will need to install better cooling equipment for the cheaper CPU.
b. You will need better air conditioning to accommodate more heat output.
c. You will need more room to accommodate the cooling solutions, so the rent per computing power goes up.
d. You will need more power to operate the CPU, the cooling equipment, and the air conditioning.
These over time can easily cause you to have a cheaper but more power hungry CPU that costs more to own than a more expensive but more power efficient CPU. It's one of the reasons that Intel is crushing AMD so heavily in the server market.