The comparison does not list the X5355 2.66GHz processors. Have a look at this link which shows a 2.66GHz Dual fitted with the X5355 processors, a jump from 4952/9436, a 92% increase:-
http://www.waitingoutside.com/2010/02/mac-pro-upgraded-with-2-intel-xeon-x5355-p rocessors/
I guess the bottom line is how much you have to pay for the CPU's. Fitted a pair of X5355 to my Mac Pro 1.1 2.66GHz and the jump is of course tremendous. Like you my machine originally had the 2GHz processors and I went to 2.66GHz Dual and then to the Quads. I did it myself, but depends how comfortable you feel delving inside your Mac Pro. In short, removed hard drives, graphics card, memory riser and removed four screws from memory cage to loosen the cage, jigged the heat sink covers to remove, undo one screw and prize front fan with the assistance of a narrow tool, or a cover from the PCI-E section.
Whilst apart clean the fan and front cheese grater inlet.
Top remove heat sinks really need a size 15 Torx driver. Purchased one from the States for $10, and it is 18" long and handcled the job no trouble. removed heat sinks and original processors. Cleaned up new processors with an alcohol swab wipe, applied Arctive Silver to the CPU, replaced heat sinks and reverse of dismantling. About an hour's work and if had to pay, about the $100.00, but if you are going to get it done, get a quote first.
The only problem, if it could be called that, is Apple System profiler in About this Mac describes the Processor as 2.66GHz Unknown. The hardware Overview shows eveything correct, 8 cores, 2 Processors each with 8MB of L2 cache.
Purchased my X5355 from eBay Australia for $351.00 the paid which I considered worth the price, but that is up to the individual.
Anyone help with changing Apple System Profiler About this Mac to change Unknown to Intel Xeon Quad Core?