How can I upgrade MacPro Nehalem octa core 2.26 Ghz to MacPro octa core 2.66 Ghz ? It's only to change the processor ? Can I use Intel Xeon (quad core) X5550 2.66Ghz 8M LGA1366 - PN: BX80602X5550 from Intel or have to buy from Apple ?
Second, yes, as mentioned in the other thread, you can use these processors and no, they don't have to be from Apple. In fact, Apple doesn't sell replacement processors.
Information on how to swap the processors can be found on anandtech.com.
AFAIK Apple does not sell CPUs but yes, you can buy the X5550 from Intel or any resellers and put it in your Mac Pro. Remember that you need two of them, you can't have one 2.26GHz and one 2.66GHz.