:?: Last time I checked, the Performa 6xx were 68k machines… and the iMac is a PowerPC G3! Beginning with System 7.1.2 on the first PowerMacs, the System uses an emulator to run 68k software, and makes it completely transparet for the user. The opposite is of course not true: you won't be able to launch software written specifically for PPC Macs on a 68k.
So newton, don't worry! You'll most certainly be able to use all your software under Mac OS 8.6, apart from some very early software written for System 1.x-3.x.
Edit: porter was quicker, and is indeed right!