939 mobos run 939 Opteron, and 939 Opterons are like Athlons, just more "polished". 939 Opterons are quite easy to find.
There are both 940 and 939 Opterons. 940 are mostly used for servers. People get 939 Opterons for their overclocking ability, as the also run cooler and use less power than Athlons.
Thanks mate, you've just made my next upgrade decision more difficult (too many platforms & and need to change to PCI-E) , had just started digging on the AMD website and now realise that the 2xx opterons are the 940 socket? as they are meant to run 2 way.