AFAIK there is no x2 3500. There was an x2 3600, but should be an AM2 part. (I heard rumors of an x2 3600+ for S939, but it was OEM only.) The X2 4400 was both S939 and AM2.
Mastermind, what do you want your computer to do? S939 is pretty much dead. DDR1 ram is expensive, and your limited with what CPU will work with it. Selling it and moving to AM2/S775 is a good cheap way to upgrade. If your not opposed to used items, there can be deals found on forums right now.