Provided the interface is x16 you shouldn't have any issues.  I mine with a HD5850 on a Dell 4700 circa 2005.  I did have to replace the power supply but the mobo worked fine.

Beware that some mobos made around 2005-2006 were known to have a bad batch of capacitors and the increased heat load from the card may be enough to do them in.  I'd inspect all caps for bulging/splitting before starting up the rig.