Hi
I'd check a certain auction site for an idea. As that's probably the best way to dispose of spare kit.
The 4800+ (assuming socket 939 by the motherboard) are still going for silly money (as above). Ludicrous really the money I've seen them go for, considering (if you look around) you can still pick up a new Opteron 180.
If you had enough posts, you could use the members market here.