Fastest GPU for ultimate early 2000s gaming rig

[LOLZpersonok]

I have an early 2000s computer that I've turned into a gaming rig for games of the same era, since some of my games don't like either Windows 10 or multicore CPUs or something else about my main system. I could use a virtual machine, but that would take out some of the fun, now wouldn't it?

 

This computer has a Gigabyte GA-7N400 Pro2 motherboard that doesn't have any PCI-e slots, it's 8x AGP 3.0 only. Currently, I've got an ATi Radeon 9200 GPU with 128MB of video memory. Not bad for most of my games, but I'd like to do better. Considering the CPU (which is an AMD Athlon XP 2500+ overclocked from 1.8GHz to 2.1GHz - Windows reports it as an Athlon XP 3200+), what is the fastest GPU that'd be appropriate for this system? The CPU lacks the SSE2 instruction set and I can't upgrade to one that does, which might be important as I've heard that the drivers for some later cards require SSE2 support.