If you can get a similar clocked x2 at 2.4ghz for less money, I'd go for the x2.
Opteron 180 costs 150$ or so on newegg, for 85$ you've got an OEM version of the x2 4200, that runs at 2.2ghz versus the opty's 2.4ghz. So if you've got a cooler, which I suppose you do, you could buy the OEM version, and save half the cash and only lose 200mhz.
I'd buy a cheap x2 3800+ toledo core and oc it to 2.4-2.6ghz though, in fact, I have allready and it hasn't let me down so far
