NJRoadfan's comment regarding Speed Doubler is important. If you are using a PPC601 upgrade card in a 68K Mac or an X100 family PowerMac, Speed Doubler will substantially improve the performance of 68K emulation.

The 68K emulation in the later x500 PCI PowerMacs was improved so that it was about as good as Speed Doubler (I think Apple actually licensed the code from Connectix, but the memory is vague).

But for the early NuBus PowerMacs and the PPC601 upgrade cards, which have what amounts to a PM6100 ROM on board, Speed Doubler is really a must.