A core 2 duo, but then you would need a new mobo to support it since you have an AMD... Currently Intel processors are the best...
I have an AMD 3500 as well Yeah waymon is right, Intel Core Duo are leading the pack right now, and the decent chips are pretty good value for money too. I just bought an e6750 - 2.66ghz for $242 (I live in Australia) and a quad core q6600 2.4ghz is about $100 more than that. But yes you need a new motherboard, and consequently new RAM as the intel 775s mostly (if not all) use DDR2. Probably looking at a new power supply too but it's worth it to upgrade from the good ol' 3500 to something more mighty.