Any 4x or 8x AGP graphics card, probably best with a GeForce4 Ti or Geforce FX if you like nVidia, or a Radeon 9800 or X800 if you prefer ATi.
Might have to be a low profile card though as its a business system, in which case, you could have difficulties.
No point in whacking a nice modern graphics card on there, even if you could get one that works over AGP (the 66Mhz AGP bus would be a bottleneck) the CPU/chipset/memory speed would create a bottleneck for firing the data over the GPU for processing.
