For an AMOS equivalent for Mac, PC, Linux, AmigaOS 4(prereleases) and MorphOS you can look into
sdlBasic or wait until the Mattathias BASIC compiler is done. sdlBasic has a very flimsy experimental compiler called b2cpp that probably isn't worth your time if you want to do serious stuff. Also the syntax is different on sdlBasic than AMOS while Mattathias BASIC is going to use AMOS syntax. If you have an emulator you can use AMOS itself on any of the platforms supported by a UAE distibution (PC, Mac, Linux, MorphOS, AmigaOS 4(prerelease), etc.)
As for AmigaOS 4, it is promised to be full release by the end of this year IF there is supported hardware to run it on. MorphOS will run lots of Amiga software but it isn't AmigaOS. But as CountZero pointed out, neither of them will run on a PC but require a non-Mac, PowerPC-based computer.
The A600 is kind of the runt of the litter as opposed to other models like the A1200 of the same time period but you can use the ROM image out of it to get WinUAE to work on a PC to play all of your old games. The A600 came with a Kickstart 2.x ROM but to run most of the internet software you'll need Kickstart 3.1 and I'd reccommend Workbench 3.1 with a 40 Meg hard drive since 3.5 or 3.9 would require a second hard drive for your data and a CD-ROM to install with.
-edit- Whoops, I misread the original post. If you want to hook a 40 Gbyte hard drive to your A600 you'll need AmigaOS 3.5 or 3.9. You won't be able to run AmigaOS 4.0 on your A600, though, so I'd consider either waiting for the official release of AmigaOS 4 or getting MorphOS to run on a Pegasos 2.
