There was talk about how 800 series AMD chipsets would theoretically run AM3+ CPUs with a BIOS update, but it would not be supported by AMD and was up to the motherboard manufacturer to determine if it was possible and if they wanted to do so .there are also some 700 and 800 chipets being sold as AM3+ boards, usually denoted with a G following the chipset number.
So you probably can't run an FX chip if you have a board that was sold as AM3, but just go to the manufacters site and check the BIOS updates and CPU compatibility lists to make sure