I'm assuming the limitation is due to the motherboard you're using, since the AM3 chips are going to be much more difficult to find than the AM3+ chips. Knowing your motherboard model would help in selecting the best AM3 chip to suggest.
That being said, you should be considering a Phenom II X4 940 to 980 (Black Editions) for a quad-core CPU, or the X6 1075T/1090T/1100T (Black Edition) for a hexa-core CPU. Going with the Black Edition CPUs gives you the option (depending on your motherboard) to also OC them.