It really comes down to environment that the processor is going to be work in. With the Intel® Xeon® E5645 you are looking two if these running on a server. In this environment the more threads that you can handle is generally more important than the overall speed of the processor. With the Intel Core™ i7-990X Extreme Edition, it is designed to for a desktop/workstation environment where high clock speed and lots of threads from a single processor is more important. In the end it all comes down to what you are going to be doing with the processor. You can see how these processors stack up against each other at http://ark.intel.com/compare/52585,48768 .