installing a linux dist

I would only try to install Gentoo if you're a seasoned Linux user, otherwise prepare for hell.

Assuming you have an Intel processor, I'd recommend Ubuntu or Kubuntu (depending on which desktop environment you prefer), as the grub bootloader will make switching between OS X and Linux very easy. They all so have immediate access to the excellent Debian repositories. If you're running a PPC (e.g. a G4), check out Yellow Dog Linux; it has better PPC support.

I've had no experience with Linux running on Parallels; sorry I can't offer you advice on that point.

 

You are talking here about both Parallels and Boot Camp. Which is it you want?

I found it was very easy to install Ubuntu inside a VMware virtual machine. Just put in the CD or point the virtual CD at the ISO image and the installer "just works". I have a 2.16Ghz iMac with 2GB RAM and it runs well

However almost everything I run under Linux will also run native under Mac OSX. Both Linux and OS X are after all UNIX varients.

3.0 Ghz clock speed will not do much. I doubt you'd notice even a 20% bost in clock speed. Mostly the CPU metr is not running at 100%. When it is less then 100% a faster CPU does nothing for you.