Decided to take the plunge after picking up a DP 4,1 a few weeks ago. It was the base model 2 x 2.26 8 core machine, and came equipped with 12GB of RAM. Immediately, I ordered 6 sticks of 4GB 1333mhz RAM. Now the machine had 24GB. So far, so good. I had a single X5690 in another 4,1 (a single processor model) that I decided to pirate, and then ordered a second on eBay for $100 shipped. After about 2 weeks, the new (to me) X5690 arrived from China, and I began the teardown. Got the tray out, and removed the heatsinks. I didn't know that there were small spacers on top of the CPUs in the DP setups. Interesting. De-lidded the pair of X5690s using the vice only technique. This process took less than 5 minutes, and was super easy. I had 100% success with this procedure, and it was my first time doing it. Highly recommended to anyone on the fence about de-lidding their own CPUs... DO NOT pay someone $70+ to do this for you! --- Post Merged, May 15, 2018 --- Using a razor blade, I carefully removed the gasket and thermal material from the processors. Then, cleaned them up with isopropyl alcohol, and gave them a quick blast with compressed air. Installed the CPUS, put the spacers back on, and began to tighten the heatsinks. I tightened them in an X pattern, switching screws every few turns until I felt resistance. Then, I gave them an extra 1/8 turn and started on the next heatsink. After that one became tight, I went back to the first one and they had loosened up a bit, so I gave them another 1/8 turn each and repeated the process to the second one. Keep in mind, I used very little effort to tighten these screws as I had heard horror stories. Finally, everything was done and put back in the Mac Pro. Powered up the machine, and was treated with a reassuring bong. "About this Mac" recognized both new CPUs, and the RAM had been bumped from 1066mhz, to 1333mhz. Immediately, I fired up Geekbench and ran the CPU test. For comparison, here's what the machine did before the upgrade (2 x 2.26ghz 4 core CPUs): And here is what my single processor Mac Pro scored with the X5690: And finally, the results of the 2 x 3.46ghz 6 core processors: So, my single-core score increased about 900 points, while the multi-core score leaped ahead by almost 12000 points. Unfortunately, the single-core scores were lower than the single X5690, but I did increase the multi-core score over that setup as well. All in all, it was a worthy upgrade that I wouldn't hesitate to recommend to anyone with a 4,1 or 5,1 machine. This will undoubtably increase the lifespan of this computer. Anyone need a pair of E5520s?