Well Opterons weren't intended to be used as we are using them, however they work perfect for our needs. If you're not that experienced, not all motherboards will be seemless with the X2, so the same can be true for the dualcore Opterons.
The 165 is pretty much out of stock everywhere you look (and if you can find one, it probably won't be for the killer price everyone has been so pleased with - $300), and its such an attractive because it can overclock just as well as any dual core AMD chip out there, and it even has the extra cache the Toledo X2s have. However the 165 comes with a 9x multiplier, which means you're going to need a motherboard capable of pushing high HTT (288 for 2.6GHz, 300 for 2.7GHz), and one board that consistently pull that off is the DFI Ultra-D.
But that brings us back to the OOS (possibly even "discontinued") problem of the Opteron 165. However if you were willing to fork over for the 4400+, you might be willing to grab an Opteron 170 or 175. However I think the 175 (same specs as a 4400+) runs a little more expensive.
That being said, don't go Opteron unless you're planning on overclocking. If you are, I'd go with the slightly slower (slower than a 4400+) Opteron 170, it has a 10x multiplier so you don't have to worry about the motherboard as much as you would with a 165. It can be found for $400 (or even less if you go with OEM), so its cheaper than the 4400+, has the same ammount of cache, and should overclock just as well (if not better).
The senario is different if you're not considering overclocking.