Don't even think of using AmigaBasic - it really is a dead-end route. Since all BASIC compillers use pretty much the same syntaxes, just pick up AMOS or Blitz. I was always an AMOS person, but I've heard that it isn't as system friendly Blitz.
Blitz is being developed as commercial s/w for the PC, but go
to www.blitz-2000.co.uk and you will find an updated version of Blitz 2, called AmiBlitz2 .
Another alternative is
PureBasic, very similar to Blitz. It has support for PPC, and other OSes (Linux, Windows). It's not free though, but you can get a great demo.
AMOS, as Atheist said, is released for free. There are a few people developing AMOS-like languages - have a look in Sourceforge if you are interested.
Blitz would probably be more OS4 compatible, but that's just my guess.
