Fedora, openSUSE, mint, Ubuntu, Kubuntu, xubuntu
Those are the "regular" ones that are relatively painless to install and use. I own and have running (mostly in virtualbox) all of these and all offer x86_64 versions(which I use). Of these, I would recommend openSUSE or Fedora.
edit: I have heard some good things about sabayon. I have never used it (always used gentoo which it is based on). It is supposed to make a source based install easier.