I have tried multiple ways of installing RoR on my macbook. Finally I went with: http://locomotive.raaum.org
Hassle-free, and works perfectly for me.
Some extra programs I use are: TextMate for coding and VirtueDesktops for switching between code and browser.
Good luck.
Macs don't really come with a lot of extra crap like a Dell does. The pre-installed OS should be fine.
For an editor, I've been using TextMate (http://macromates.com/). I must have got a discount, because I don't remember ever paying ~$50 for it, but it's a great editor for any task.
I haven't run RadRails (http://radrails.org/) on Mac OS X, but it also is a great environment to work in.
Stephen Gerstacker
Then let me be the first to simply say: “No” 
A Mac doesn’t come with a heap of crap applications like some PC suppliers (ahum Dell ahum). Should you get into disk space problems, you could remove GarageBand (and it’s related files in Library > Application Support) if you don’t use it, but with hard drives nowadays, that probably won’t be necessary in the first few months :-). You can also skip a virus scanner and spyware/adware remover, so for the first time in your life, your portable won’t lose 10% of its processing power just to keep your mind at ease.
I can recommend you some of my Mac favorites:
• TextMate (the must have Rails editor if you are on a Mac)
• QuickSilver (http://quicksilver.blacktree.com/)
• iTerm (Terminal with some extra nifty features)
• Transmit ((S)FTP)
• Vienna (RSS reader)
• Adium (Instant Messaging)
If you want to find these apps, check out http://www.versiontracker.com/
Best regards
Peter De Berdt