You'd be best off getting something like an old A500, like da9000 said. That way you can find out about how the OS works, and what games there are for it.
I wouldn't buy an AmigaOne until the hardware is in production and AmigaOS 4is released.
Alternatively, you could upgrade an ol amiga. I recently bought an A1200 and a 50MHz 68030 accellerator card. That makes it a lot faster. Combined with the latest OS, AmigaOS 3.9, it should be great.