IMO, are you sure that your motherboard is only AM3 compatible, and not AM3+? It's been a while since I've seen a board that actually didn't call itself AM3+, even if you would need to do a quick BIOS update. If that is the case, you could look at some of the new FX chips that will still meet the TDP limit. The only case I can think of where this would be unlikely is if you have a Dell or other OEM system as a platform. In that case, I'd go for the Phenom- as someone else said here, better architecture.
Sounds like an OEM computer. If it is. You can pick up one of those true quad cores but the performance will be so similar to your Athlon X3. If you have a regular mobo you could try unlocking the x3 into a quad and overclock it and get the same general performance.
A waste of money to be honest if you have an HP and so on. I made that mistake on an OEM and the money went down the drain.
Use that money towards a better Graphics card IMO.