I wouldn't waste your money on old AGP, although something like a AMD (ATI) 3850 or 4670 are possibly the highest cards I've seen with AGP (may be wrong).
You would be better off upgrading your CPU/motherboard/ram and getting a PCI-E graphics card though in the long run.
Honestly I think you should save up for a brand new system instead... That P4 will bottleneck the 3850...
what does 'bottleNeck mean mate'
edit.