3870, 3850 and 4670 in that order are the best for AGP ... but as others have mentioned, usually any of them are going to be overkill for the CPU, since AGP machines tend to be single-core P4s. A 4650 or even a 3650 is going to give roughly the same results with less power consumption.
I don't agree with the mentality of "it's not worth it to upgrade a machine that old" 100% of the time. Yes, this is never going to be a good machine for modern high-end gaming. But it can make perfect sense to get an AGP 4650 if you want to keep it around as a "second" or "backup" computer. My own household would have serious difficulties if we only had one machine.