I just installed an Athlon II in place of my Athlon 64 x2 CPU. The computer would not boot, nor would it even POST. Totally incompatible. Very much like trying to use a Core 2 Duo in a Dell Optiplex GX620, which will also simply not work.
Bummer. Oh well. I post this so you will avoid the hassle. I couldn't find a clear answer to this question so I did the original research for you.