If you want to use PCI card look for `Fujitsu Siemens Futro S400`: https://www.parkytowers.me.uk/thin/Futro/s400/
It has the same CPU and chipset as HP T5720 (tested by Phil) but it is IMO a better choice because:
* Case is big enough to install one of these chinese GeForce FX5500 PCI
* You can install CF card inside, additional adapters are not needed (16 GB Sandisk card didn't work for me, but 4 GB Transcend was fine)
CPU performance is on level with P3 or Athlon 1100, so it should be more than enough for games released till 2002/3.
P4 3.4E, P4C800-E Deluxe, 1 GB RAM, X800PRO 128 MB AGP, SB Audigy, XP SP2
XP2000+, KT2 Combo, 512 MB RAM, GF4Ti4200 64 MB AGP, FM801, Me
PIII 550, ABIT BH6, 128 MB RAM, Voodoo 3 2000 16 MB, YMF719, 98SE