These questions are valid. Luckily, Apple has done an amazing job preparing for the transition to RISC, especially in terms of backwards compatibility. Existing software for the 68000 processors is run in emulation by the PowerPC processor. Performance on programs not requiring extensive floating point calculations is comparable to the high-end 68040 machines. But you can expect significant speed-ups when the slew of native mode Power Mac applications currently in the pipeline arrive this spring and summer.
Stability running older versions of existing software is very high.
Apple estimates 90 to 95 percent of the software out there today will run with no problem on the Power Mac. This was also confirmed by an unauthorized test conducted secretly by MacWEEK magazine. MacWEEK staffers obtained a pre-release 6100 CPU and put it through a series of abuses that most machines would never be subjected to. Countless system INITs and CDEVs were loaded onto the system folder with nary a whimper. The machine scored a resounding approval rating from the MacWEEK crew.