You migt want to seach this list for previous posts on this subject, but this works for me:
Fedora Core 6
Ruby 1.8.4 or 1.8.5
I found this easy to maintain, ( I’m not a Linux guru, but, am still required to maintain my own dev boxes).It’s more stable and faster than WindowsXP. I will probably use this same basic setup when my current app goes to beta. There are other great platforms that people on this list are using. I seems to be more a matter of preference. You could even use RoR from plasmacode on WindowsXp with tortoiseSVN and InstantRails.
Good Luck and happy coding