@melott
re: Amithlon
There is no demo disc. The link you saw in this thread is just a hardware compatibility test for your motherboard and gfx card. It doesn't check your sound and network card compatibility. It's also quite old.
Amithlon is a program that runs on a bare-bones custom Linux kernel, so strictly speaking Linux is the OS. However, this is transparent in operation so the effect is that you'd be running AmigaOS on an x86 system. (You can't run Linux applications on it)
It uses off the shelf graphics and sounds cards, but the supported range is limited (much more so for sound cards than gfx cards). You really need to check hardware compatibility first.
To add to what Kronos has said regarding the legal status, while it is true that nothing is ultimately proven one way or another, the dispute has affected support and upgrades. At this time, the product appears to be at a dead-end (an upgrade has been promised for many months, but so far it is still MIA), while some of the unofficial upgrades that were released have been retracted and are no longer legally available for download.
It's something worth considering when deciding whether to shell out what is a considerable amount of money.
What applies to both Amithlon and UAE is that you need to check your CPU. 500MHz doesn't mean much by itself. Is it a Celeron, a P2 etc, or is it a K6-2 or a VIA chip of some denomination. If the former, you're fine. If the latter, I wouldn't hold much hope.
