Picked up a cMP 4.1 with a single 2.6 ghz w3520 CPU earlier this week. It passed numerous stress tests I threw at it using Geekbench. I was a bit worried as I bought it well below average market price here, so thought something must be wrong with it! 5 days now and so far so good, touch wood. Got excited, did the flash to 5.1 firmware, installed 32 GB of 1333 mhz RAM. I was obsessing over CPUs all week, found a deal on a x5670 (2.93 hex core) which I just received and installed today. Seems to be working, just running a few more stress tests, hoping for no errors, fingers crossed. Just wondering what CPUs other new 4.1 and 5.1 owners are throwing in at the moment? Ideally I wanted an x5690 but they are close to 300 UK pounds or US $300-400. A x5680 at 150-180 UK pounds or $200-250 are also quite pricey, so I settled for the x5670 which was again half the price of that. My x5670 is getting around 5-10% less performance then the x5680 according to Geekbench. For my use that is fine, and the extra 5-10% performance is not worth paying double. These things are so much fun, such a shame Apple stopped making them like this.