If you plan to make it do "serious stuff," then get a G3. I couldn't imagine using my 1400c without its G3/466, but I did pay dearly for it.

If it's a machine to just play with, even the 117MHz card isn't bad (I made do with it for a number of years before getting my original G3 upgrade), and I second the recommendation to just stick with the 133MHz card rather than foof around with a logic board swap because the cache makes more of a difference in the 1400 than the CPU speed on the low end (the slow bus, among other things).