You can usually just use 8x cards... But that would be boring.

I think the Radeon 7500 was AGP 4x and it was an awesome card. Some of the earlier GeForce 4 Ti cards were AGP 4x I believe. Which is faster is debatable and always has been, depends what you try to run but generally they will fare fine in period correct applications and have their own different pros and cons.

Any 1.5V AGP card should work on that motherboard. Voodoo3 will not, as it needs an older 3.3V AGP slot (and unless you're using something like a SiS chipset, yours is probably 1.5V only).
The most powerful, and with decent Win98 compatibility, should be something like a GeForce 6800. Unless you, for some reason, want Pixel Shader 2/3 compatibility and good performance, though, you can settle for a higher end GeForce FX.

Also, people mentioned before that ATi had more problematic support for WIn9x with their later cards, so do some research beforehand. If it is a Windows XP system, though, you can just use whatever fits the slot.

I had an geforce 4400 back in the day, the 4xxx series was AGP 4x in earlier revisions and then the later models went to 8x.

The 8x cards work in 4x slots, just at a "reduced" speed.

is there a windows 3.11 driver for radeon 7500?