Well a cheap 68020 with Fast RAM will allow OS3.9 to be installed and to run WHDLoad.
My first CPU card for my A1200 was an overclocked 68020 (28MHz) with an extra 4MB of RAM and that thing satisfied me for years.
We have a winner. The Whdload bit. A500= games machine, so why bother with more than 8 meg and 15 mhz '020
If you do anything that needs more horsepower than really just get an A1200 and expand that. As a bonus you get AGA...
