What does this cost? At the crossroads. Only room for one.

[ElPolloDiabl]

Check out the FPGA arcade. It looks like Mike is just about to start a big production run.
The Amiga hardware is getting quite old now. I personally wouldn't buy one unless it was very cheap.
Also are the upgrades you might want available and what would they cost?
If the FPGA Arcade doesn't interest you, go for the AROS box and have lot of fun with it. AROS gets drivers faster than any other next gen system.
G5 sounds good too, if you like the Macs.
A1000 it's for rich collectors and people who don't mind waiting years on end for software to be released.

[ElPolloDiabl]

I might second the above arguments of getting another hard drive instead of another machine.
  I've always had problems setting up dual/multi booting. If the machine crashes and you don't have a backup it is a lot worse than a single install.
  However with all the cheap external drives you can get nowadays you can set the external drive up as a primary with an OS on it.
 Then change the bios to let you boot from USB or eSata. Then select to boot form the external drive at the start up.

I installed AROS once on some older hardware without any problems, I haven't bothered to try again, but will soon. If it can do everything Linux can, then it will work nice for a basic system.