iirc the a3620 board uses a PLCC type fpu socket. The 68881 may be overclockable to 25MHz but you'd be better off just finding any old 68882 and putting a 50Mhz crystal directly on the cpu card and enabling the jumper for the separate fpu clock. Most if not all 68882s should operate at up to 50MHz w/o problem (I have a 40MHz 68882 overclocked to 50MHz on an old A2620 card in my A2000 and it works great). The fpu will speed up certain math-intensive operations that utilize the fpu libraries. Better yet, just find an old A3640 card with a full 68040 chip and you'll have a much faster fpu than any 68881/2 ever made (though the tradeoff is you'll find motherboard RAM access speeds are slower with the 68040 than the 68030 you already have).
It is possible to overclock the motherboard but you're asking for trouble as you'll lose Zorro slots or other timing problems will pop up. I never did it myself but ask redrumloa about it; he was able to speed up the AGA display by overclocking the motherboard, iirc.
