On the ATI side, I've been happy with a Radeon 9800 Pro on an Athlon system. I never thought much of the nVidia FX5000 generation. The AGP nVidia 6000s might be pretty good, I've never had one but I remember good reviews for the 6600GT.
In case you're interested, there's an old review, I think from xbitlabs, which measured the power consumption of video cards from that time period. It showed that the Radeon 9000 generation were the last of ATI that depend mainly on 5V and 3.3V power. Once you get into the "X" range they mainly use 12V. So if your power supply is already set then choose the card accordingly.
I don't know when the change to heavy 12V happened on the nVidia side.