An AM2 motherboard? Well, you should worry more about the bottleneck instead. IMO, as long as the mobo supports PCI express cards, then it definitely will support the 6850. But using an obsolete platform for the latest GPU is like installing a turbo on a 1000cc car. It will run, but not at its peak potential.
My advice, upgrade to a newer platform if you're planning on using the 6850. But if you still insist, a simple video driver update and confirmation that your current PSU is up for it are the two things you need to do.