The 2.66 is the most economical option [not the cheapest]. Arstechnica calculated that the 2.66 MP would cost about $2400 to build on your own [not including OSX, iLife, the case and power supply which could add $300 to the cost of the machine in hardware alone]
Xeon 5150 [2.66 Ghz Xeon in the MP] retails for $800 each. The 5130 [2.0 GHz] retails for $400 each. The downgrade only nets you $300, not worth it in my opinion.