I recommend Ubuntu... Such a spectacular OS...
But watch out... Ubuntu 7.04 came out about a week and a half ago I believe, and there is this nasty bug on it...
If you have more then a single drive on an IDE chain, it will refuse to boot into the LiveCD so you can install Ubuntu itself... There is a workaround to this, and even me not being a linux guru, I managed to follow some steps..
1.Primary IDE to your HDD you wish to install Ubuntu, Secondary IDE to your CDROM
2.Install Ubuntu, it will let you patition, etc....
3.Find the fix online... (I can do this if your serious about it) it just has you open up a file and type in some needed lines.
4.turn off PC, reattach all your drives, then the next time you boot, it should be fine.
The bad thing here is, how will it know the other installs of other OSs on other drives... If you have windowsXP on the same drive your installing Ubuntu, then you are ok.
Either then that, Ubuntu linux was the closest experience to an Amiga.
I have it installed on my 2nd PC, 1.2ghz, 512mb pc-133, 10gig drive.... And wow, after it boots, its a breeze through apps.