@barney
A 68020 is hardly fast enough for... almost anything these days, not only for AOS 3.9. A memory expansion board with some 60ns Fast-RAM on it is a good investment, independently on if you decide to install AOS 3.9 or not, as it will increase system performance and it will allow you to run all those good speed-up patches you can find on Aminet.
I recommend a 68030 accelerator instead of just a memory board.