Hi TimHi there,
Do not get the Cyrix chip for your bridgeboard (C= or Vortex). Get the TI 486 SLC2 50 Mhz. The difference is the Cyrix has a 1k cache and the TI has 8k, performance difference is significantly increased.
There is a seller on eBay who has them. You will need to solder the chip directly to the board. There are a few AmiBayers who can do this for you if you don't have the tools.
Run the chip only with hestsink/fan as it will get got. Chip is 5v tolerant, but the core is 3.3v resulting in higher temperature.