Ignorance is a bliss.
I'm fairly sure that the highest FSB available for a socket A CPU was 200mhz, which is not miles away from 166mhz. You probably will have to update your BIOS in order to buy a new CPU because the only socket A models still available easily are Sempron, which will require a BIOS update.
If you buy a second hand CPU just get one with the highest speed you can afford - the FSB speed tended to increase with the speed of the processor - if you get a 3000+, for example, it'll likely be 200mhz.
Just buy the fastest processor, and update your BIOS.
Try bouth things.