It depends on what you are using your Amiga for. For mostly older Amiga software and games a faster 68030 works great. For more modern computer needs a fast 68040 or 68060 will allow you to do 90% of what most people use modern computers for. For some CPU intensive tasks the classic Amigas are just not up to par. A G4 PowerPC Amiga is probably fast enough for 98% of what modern computers are used for. The lack of good modern stable Amiga software becomes more of a concern with the faster processors IMHO.
