These images are of my recent project - upgrading a 2006 Mac Pro desktop with two new CPU's!
The original Mac used two Dual Core Xeons (total of 4 cores) running at 2.66Ghz. It's possible to use two Quad Core Xeons running at 3Ghz (total of 8 cores). This brings the specs up close to a 2010 model Mac Pro - basically doubling the "Geekbench" scores.
This has been an available upgrade for a number of years, but the price of the Xeons has been a little prohibitive. Most people used slightly slower Quad Core Xeons as they were better value. Now, however, the faster Xeons are much more affordable - I got mine for NZ$100 for the pair.
There are some great YouTube tutorials available for this upgrade. One of the photos shows my laptop (with the tutorial) alongside the Mac Pro as I did the upgrade. Everything works as it should so far and the temperatures are staying low thanks to the use of Arctic Silver applied as suggested in the tutorial.
Look out for my eBook on disassembling and selling computer parts, and maybe one on how to do thje same for Apple desktops and Laptops on iBooks!