I actually own an Intel system with the E4500, another system with the E6600 and a system with the X2 5600+ The E4500 is a little slower than the 5600+ which is a little slower than the E6600. The E4500 runs the coolest; the 5600+ a bit warmer and the E6600 the warmest. My understanding is that the E6550 runs much cooler than the E6600 and provides a bit better performance.
The 5600+ is the only processor that I would consider buying and I won't buy one right now as there is no upgrade path. The Intel processors have upgrade paths right now or have overclocking headroom. My understanding is that the 5600+ doesn't have much headroom and then only at the expense of a lot of power.
The newegg market focuses more on the enthusiast. While it's a lot easier to make comparisons on Newegg because of the level of price information, I prefer looking at comparable models at Dell and HP. At the moment, a system with an E4600 is $50 less than a comparable system with the X2 5600+. The 6550 system is the same price as the X2 5600+ system. Given this, one who knows the price, power, performance, upgrade paths and headroom would go with the 6550. For another $50, you can get a quad 6600. I think that it's pretty hard to justify buying a 5600+ from Dell; which is why I recently bought an E4500 from Dell.
The Dell consumer will probably not pay as much attention to power considerations as the Newegg consumer. But it's pretty obvious to someone that thinks a little. I think that you'd be very hard-pressed to find a Dell system with the X2 6000+ in it. But you could buy an E4600 and determine that there were two faster processor options for the same box which should indicate that the box has the power capacity for higher performance processors.
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