Moved to this thread to consolidate.Concern: Some say if you have the 2.26GHz "Nehalem" Mac Pro, don't upgrade to a processor with higher power consumption. The Nehalem is rated at 95W, and the 2010 Westmere at 130W. One post says the best upgrade for the dual 4,1 is the 5675. (A pair costs only around $100 now.) Pretty sure I've read accounts of people using 5690s in the 2.26, but I'd rather play it safe.
That post also recommends against de-lidding and provides what looks like a pretty straightforward solution: a 5mm thick thermal pad on the bottom of the heat sink; two 1mm thick washers on each post; and removing clips from the fan connector.
Bob
De-lidded CPUs allow the 4,1 dual to function as engineered and are the best way to go IMO. De-lidding eliminates the risk of bent CPU socket pins; the washer approach does not entirely avoid this potentially costly problem. De-lidding also allows a more direct path between CPU and the Mac Pro heatsink, as originally engineered by Apple. That said, many 4,1 > 5,1 upgrades have been performed using lidded CPUs, both with and without the washer stack. Thermal pad addition is definitely required if IHS is not removed.
I'm pretty sure the successful and trouble-free X5680/90 dual CPU Mac Pros number in the many thousands, including a significant number of MOTUNation members; concerns expressed by the referenced poster are, IMO, exaggerated.
As a DAW user, I'd go for the dual hex and de-lid. But that's just me...