I will say it like this..
If you have the Single Quad-core 2009, flash the 4,1 to 5,1 firmware... get the w3680 or w3690(your choice), but bear in mind if you have the Quad-core w3580 at 3.33 Ghz, your best bet would be the w3680 as w3690 does NOT provide too much more in performance..
Thats it.. all you need to spend money on is a processor and I have seen w3680's for under 400 dollars. Forget OWC's turnkey method!
If you have a 2009 Dual system... it will wind up costing you $$$ as not only would you have to replace the heatsinks, but you will also have to get the x56xx series processors and those alone for two of them will run you over the cost of a 5,1 machine itself.
In my opinion, best to sell off the 4,1 for the 5,1 dual.. unless you know someone who has the parts you need at a reasonable price.
Single quad users like myself only have to flash the firmware and then get the processor. I don't see too much difference between 1333 mhz and 1066 mhz DDR3 memory.