According to Guru3d's review of the chip.. "you can use these processors on an AM2 AMD 790FX/GX motherboard just as well. In fact even AM2 780 motherboards should be able to run them" and "Next in line is of course the Phenom II X2 550 BE processor. If we were to x-ray the processor, you'd spot 4 cores as I'm 100% certain that this is the Deneb quad core based architecture with two cores disabled."
The screenshot from CPU-Z points out that it is indeed a Phenom II 940 with two cores disabled, which is an AM2+ socket chip.
Even though the Phenom II X2 550 is labeled as AM3, I am looking for confirmation that this will indeed work with an AM2+ motherboard so I do not end up buying something that is useless.
That's the selling point of AMD's processors. You can get an AM3 board and add DDR3 ram, or use an AMD2 board with the old DDR2 ram. It will work, just like it has been for all the other Phenom II tri and quad cores.
The 550 is just a Phenom II chip with two cores dissabled. In otherwords it didn't make it as a 955, 945, or 720. The 940 and 920 are the same, but for some reason was released as AM2+. I suspect there was an issue with the DDR3 controller early on but they must have fixed it. Anyway if your board supports the other current Phenom II CPUs, then it should boot up just fine with the 550. It may not be properly recognized without a BIOS update though.