I picked up a used Mac Pro (2009) model, (4,1) off of Ebay for fairly cheap. ($600). It’s been a good machine, and for the price, quite a bargain. The stock CPU (2.66 Ghz) has been decent. After doing some research, I found out you can replace the CPU’s quite easily. Once again, I went back to Ebay and found a 3.2Ghz Xeon that should work. Sure enough, if you have ever installed a CPU on a regular PC then you know it’s not that difficult. The only issue I had was with finding a 3mm Allen wrench that would fit down the narrow channel in the heat sink. Luckily, I have some wrenches that I purchased for working on my Mountain Bike, and it worked perfectly. You can see the gigantic heatsink placed upside down and the daughterboard that holds the CPU. Note in the background the really long Allen wrench.
Here is a screenie of MacCPU ID for the newer processor after it’s been installed. So far so good. I replaced the thermal paste with some Antec Arctic Ice. The CPU has only peaked at 40˚C. 