the specifications you give us for the mac are more than enough to use linux... (if xp works, linux will work too)
I would say, explore
distrowatch and choose a distro that fits you...
harddisk: 20 to 30 GB is more than enough to install linux (I would prefer a seperate /home partition of min. 10GB and also a / partition of min. 10GB for a major distro)
=> there are more small distro's as well
when you have a problem, ask it!
(but I personally don't have any experience with mac)