AROS (currently) builds for the x86 (standard PC), the x86-64 (64bit Modern PCs) and a few PPC systems.
What you are thinking of and I am very excited about is the work Jason and Toni are doing to get it to build for Amiga hardware. And they have got quite far... once they have done it, you will be able replace you old 3.1 OS with AROS on your Amiga 
-Edit- What do you use to post this? A PPC mac (vague memory of you saying this)?
Will the performance of AROS 68k be anything more than a curiosity? How many of AROS advanced features will be carried over to the 68k version I wonder?
I like MorphOS2.6 on dirt cheap hardware, as it runs quite fast and well, but must admit that the progress on AROS is getting more interesting every week, and unless there is a change in the rate of OS advancement, new software titles being written for, or ported to MorphOS2.x and/or an eventual port of MorphOS2.x to better hardware in a year or two, I have little doubt that AROS will eventually surpass all other Next Gen Amiga experiences. Might take a year or two to happen, but unless something changes in the MorphOS camp (applies to the AmigaOS4.x camp too IMO), AROSx64 will probably come out on top eventually. (just like I had hoped that Linux would over take Windows some day in the distant future, but that wish seems to be taking much longer than I had hoped, so maybe I will be wrong about AROSx64 too)
