For beginning Linux users I recomend Fedora or Debian. Ubuntu's nice, but it's really regarded as a n00b OS, and I personally don't like it. Mandravia is also really nice, it keeps its packages up to date better than any distro I've ever used. Gentoo or FreeBSD with portage are great for running lightweight systems, or any type of system for that matter, but are a bit more difficult to deal with.

Also, normally I'd recomend GNOME, but with such low ram, something like Fluxbox or Blackbox are probably better. If you must have a full desktop enviroment, xfce is probably better.

Xubuntu is probably your best bet if you don't care that you're using Ubuntu. Else, Fedora, Debian stable, Mandravia, in that order.