8 megs is the maximum amount of 16-bit fastram you can put on an amiga. If you get an accelerator that has 32-bit ram expansion capability, you can add alot more fastram.
As far as a 68000 based A500 goes, the only other thing you can do is max out the chip-ram to 2 megs. The easiest way to do this on a stock A500 is to get a small upgrade board that plugs in the agnus socket.. eg. DKB Megachip, Mini-Meg, etc.
I really have no idea the nature or reason for your file-transfer problems.. 8 megs of fastram should definitely be enough to run any sort of application youd ever want to run on a stock 68000.
