Stating the obvious, Ebay. I'm actually upgrading my CPU too. I'll be going from a D950 to a Q9400 or Q9550. I'll probably drop the D950 into my old 945 MOBO (swapped for a P5N-D) and either sell as a barebones, of buy a fan/GPU and use it for an Xmas gift. You could do something similar. Actually, I have seen AM2 boards for $50. You might use it for a network storage or data backup PC. Maybe even a docking station or an HTPC.
Well I'd have to sell it or get rid of it somehow - I've got the NAS sorted (my server acts as NAS + Apache + Print + Music server and more
Maybe I'll leave it on the shelf and see what happens to it
Or overclock it until it breaks
I might look into selling it on ebay actually. Although I doubt the sort of people that build their own systems would want my CPU.