AFAIK, AM3 CPU's are not compatible with AM2 mobos. They should be with AM2+ mobos, but aren't always, such as in my situation: I have an Asus M3N78-VM AM2+, but only the latest revision is compatible with the Athlon II processors. The first revision is not.
Your best bet to upgrade is to look for an AM3 motherboard that supportd DDR3 memory, and uses the AMD 785G chipset. Don't get the Sideport memory feature if you plan to add a discrete graphics card. Personnaly, I would stay with the 3 major players: Asus, Gigabyte, MSI. Since I have had a couple of good experiences with Asus tech support, I would tend to favor them.
You can have a look at the Asus page showing all mobos for the 785G chipset. (unlinkable)
Here is one that has the features I like and no Sideport memory:
http://www.asus.com/product.aspx?P_ID=LVmksAnszmVimOOp&templete=2
Inexpensive too:
http://www.newegg.com/Product/Produc...&Tpk=M4A785T-M