It really depends on what you are going to do. Really probably 16MB should do.
Getting past 8 MB on a A500 is hard even with the best expansions. 16 MB is the limit of the MC68000. So I dault this value really reflects how it works.
For the A500 you may want to try one of these serial to bluetooth adapters. I use one with my Minimig but so far I have been unable to get it to connect to windows7 via PPP which is my ultimate goal. I suspect with linux I'd have no problem.
Microsoft can't make good software nor offer sane licenses. And I prefer wired.