You won't have to replace the CPU, most likely. I've never run into a IIsi that didn't "overclock" (actually, re-clock) to 25MHz. And this is a case where the speed difference is very noticeable. At 20MHz, the IIsi barely gets everything done. In imprecise terms, an extra 25% clock speed is all used to accelerate user tasks, so it will feel much zippier.