How fast is "fast enough"

[A6000]

We have 020's at 14 mhz w/FastRam, 030's at 25 to 50 mhz, 040's at 25 to 40 mhz, and 060's at 50 to 90 mhz,

Now, putting aside the view that a computer can never be "too fast", what would be the minimum useable processor and speed.

[A6000]

Thanks for all the replies so far.
My question was related to running the sort of AMIGA software you are still running now, most of this was written for a base machine at 7 or 14 mhz and should run on that machine but a little extra speed is usually good, so I was wondering which speed is usable, I know PC's are faster but most of that speed is wasted running windows, and even that machine will be OBSOLETE in 18 months if not sooner.
A lot of people are trying to find 060 accelerators, but could they be satisfied with an 030 or 040 if they dont want to play DVD's, I can buy a brand new DVD player in my local supermarket for under £15 which is better and more economical than trying to make the amiga do it.