16 is OK if you don't use a gfx card (you will probably be able to run most of programs with the exception of the allmighty TOTAL CHAOS(tm) and a few others)
32 is quite comfortable and you'll probably have enough ram for most of stuff.
64 is IMHO more than enough for a non-gfx card equipped A1200 and will allow you to run comfortably compilers, gfx apps...
If you like unarchiving very big files in the ram disk you may want 128MB of ram but IMHO 64MB is enough for a 1200/030.
