Well, that all depends on what you want to install and what your going to be using it for. Most Distros run from minimum or around 400 megs to 1 or 2 gig's.
suse Linux can get up to around 5 gig's. Also there are some versions of linux that can fit on a floppy disk.
From your specs...you have plenty of power and space.
Best place to get linux is either local store if you want ot buy it, or download it from the distro's site.
Or you can even get it here, a good site with iso images to burn to a disk: http://www.linuxiso.org