^ AFAIK, you need either 56xx or 57xx as a minimum for Eyefinity. If it's for gaming, you pretty much need at least a 5850 just because you need a lot of rendering power but if it's just for 2D stuff a 5750 might be good.
You do need an active display port adapter, but the single link one's (like from ACCELL) is only $40 while dual link are $140. The difference is single link only supports a monitor up to 1920x1200.