You've only got a few options fopr mobo support of the Opteron 1218. It used to be with the Skt939 Opty's (100 series) that they were compatible with any Skt939 mobo that also supported the FX line. With the 1000 series Skt F it *should* still be the same way, but don't quote me on that...
Believe it or not, the AMD website has a "Develop With AMD" section that lists recommended motherboards.
No matter tho, according to the AMD website, for a decent gaming/workstation mobo, you've got a choice of the Supermicro H8SMI-2 or the H8SMA-2, the Tyan S2925, or the Asus M2N-LR. Out of those choices, I recommend the Supermicro H8SMA-2, as it looks to be a pretty nice peice of pcb.
Good luck!