As far as alternative operating systems use, you'll need more than a RAM upgrade. You'll need to upgrade the CPU to one that has a "Memory Management Unit" as *nix os's need this.
The A600 has a 68000 cpu which does not have one, neither do the 68EC020 of the A1200 or the 68EC030 of the A4000/030.
CPU's that'll work:
68020 + 68851 (The 68851 is a seperate chip), does any amiga have these?
68030, this came stock with the A3000 but AFAIK A600 '030 boards are a bit of a mod.
Not forgetting the '040 and '060, BTW AFAIK NetBSD doesn't support the '060.
Plus you may need an FPU either 68881 or '2.
Also I think they need a good amount of RAM as well.
(basically I wouldn't bother)
srg