A 4650 would be your best option as it uses little power, however any modern card will be bottlenecked by that processor so dont drop too much into an AGP card unless you are going to OC your CPU.
They are more expensive indeed. There is an article here on Tom's that showed the 3850 pretty much smoking all the other AGP cards...even the 4000 series AGP cards. I had a 3850 in an Athlon 3400+ system, and on the counter strike stress test it got an average of 140fps...not too bad considering. I could play Fallout 3 on it well enough, and even Crysis on low. All at 1280x1024.