You could start with GPU Database at TechPowerUp, which has 92 GPU on PCI cards.
The "ultimate" NVIDIA and ATI cards in PCI were GeForce GT 430 and Radeon HD 5450, respectively, but I assume you would not use those 2010 vintage cards on a Pentium-MMX build where the CPU becomes the bottleneck and you need it to play late DOS and early Win9x games, something those two cards way too new for.
Therefore, you should set your limit to those with Win9x support, namely no later than GeForce 6 or Radeon R300. GeForce FX 5200 and Radeon 9100 were probably the best PCI cards you could acquire with Win9x support, but IMHO they were still too new for your Pentium-MMX.
So you have to look even further: GeForce2 MX 400, Radeon 7000, Matrox Millennium G450, S3 Savage4 (I wonder why TPU has no S3 card in their database), or Voodoo3. The final decision would depend on your personal need (namely which game you want to play), your budget limit, and the availability of those vintage cards.
UPDATE: While writing this reply I found a thread by many VOGONS veterans: Which PCI Graphics Card For Socket 7 430tx/hx Builds?