HP Compaq 6005 SFF CPU upgrades!

[Red Machine D]

Okay, so I salvaged one of these things yesterday and it's got a weak little Athlon X2 in it. I want to use it since it seems like an efficient little bugger, but I want to put a better CPU in it. Asking HP seems to just lead to some Phenom X3s, but I have a hunch it can do better. Does anyone know which CPUs will work in this thing?