The fastest production card is the Goldengate 486SLC2/50 which is a 386 class processor (486SLC) with a cache and clock doubled to 50MHz.
The fastest card possible is the A2386SX upgraded to 75MHz using an SLC upgrade over the original 80386SX. You can change the clock on this board to make it triple to 100MHz but I don't know of anyone doing this as you'd need a 100MHz capable 486slc! You could use a 486SLC2/66 chip though and clock double a 33MHz bus.
Just to be clear though, these are all fast 386 class chips, not 486 (and you can entirely forget Pentiums!)
Prices are normally £100-£200.
