@yester64
In short: If you are interested in Amiga 68k (for retro gaming, etc) then your best option would be getting either a real Amiga from e-bay and similar, or use Win-UAE (or some other UAE flavor).
If what you are looking for is more to use "Amiga" for more modern stuff (like Internet, playing today's media files, etc), not for retro, while still being able to run those good-old Amiga 68k applications on much better performing HW than the 68k machines, then you basically have 2 options; either OS4 or MorphOS, and both uses PPC ISA but has totally different views regarding hardware support.
MorphOS supports a broad variety of mainstream computer systems in the shape of Mac PPC computers (even laptops) that is very easy to get a hold of, is very cheap, and yet among the most powerful the PPC ISA had to offer when it comes to desktop performance.
The OS4 has gone in completely the opposite direction by only supporting ultra-low volume custom computer systems that either costs $1,000+ and is outperformed by an ordinary cell phone, or costs $3,000+ and is about on par with the available MorphOS HW when it comes to performance.
OS4 is Hyperion's (a game porting company famous for porting 6 ancient (1998-1999) games to Linux (mostly)) attempt of creating a PPC Next Generation Amiga OS building on from where Commodore left the OS when going bankrupt.
MorphOS has the same idea (and was released long before OS4 for that matter, so maybe it would be more correct to say that OS4 builds on the same idea that MorphOS does) — bringing a next generation OS that has many modern improvements on top of the Amiga OS 3.1 API. Technically speaking, MorphOS is everything OS4 is, but is faster, has more features, has better features, has better Amiga compatibility, and it has all the best "Amiga standards" API's not only bundled, but *integrated* in the OS (like CGX, MUI4, Poseidon USB stack, etc). The MorphOS "Workbench" (called "Ambient") is by far the most advanced one of any Amiga flavor; needless to say, this is a very important part of the total user experience, and competition is miles (or years) behind. MorphOS also has the best web browser (Odyssey), the AROS and OS4 versions of this MorphOS browser are still lacking some important stuff. MorphOS is not only better than OS4 on almost every measurable point, it's also much cheaper than OS4 to get going, thanks to it supporting mainstream PPC Mac computers (available dirt-cheap as second hand all over the Internet), and you can try before you buy (a *full* version of the OS can be downloaded
from here, that is fully usable for 30 minutes (it's *not* a limited functionality demo version, all features are there), and after that it takes a reboot (the OS takes ~5 seconds to boot once the hardware initialization is complete). MorphOS runs on laptops, on big-box systems with multiple PCI-slots, drive bays etc, and on tiny footprint computers, yet as powerful as PPC ever got.
Simply put: MorphOS is Amiga
done right! 
