Many of the low end motherboards are getting support so you may be able to find a board within your budget. I have a motherboard with the 770 chipset and SB700 and have already gotten a BIOS update to support the Phenom II 920 and 940.
As for compatibility it seems that most motherboards with SB700 or SB750 are getting support for Phenom II and all subsequent AM3 processors. It looks like most boards with SB600 are not getting Phenom II support, or at least they aren't from ASUS or Gigabyte. I'm not sure what the level of support