It depends on the revision number you have. I had a P2B (just died :*-( ), but it was an early revision and it would not support a slot-based coppermine.
If you use a slot-based chip, then you would need to have revision 1.12 or later to support Coppermine voltage. The P2B multipliers go only up to 8.0X but since these chips are now multiplier locked, that shouldn't be an issue (though, I thought the 850 was the max on 100 MHz bus speed). If you go to a FCPGA (Flip-chip pin grid array)Pentium III, you would need a Slocket adapter (make sure it supports voltage changes like Abit Sloket III). If it supports voltage changes, then the revision of the P2B is not an issue anymore since the adapter would control voltages.
As for you question on differences in SECC-2 and FCPGA. The SECC-2 (Single Edge Contact Cartridge) is that the processor is held in that black cartridge that you slide into the slot on the P2B. The FCPGA is similar to the old Pentium/486 chips. A flat chip with 370 pins on the bottom that plugs into a socket and is lays flat.