Ubuntu 9.04(Jaunty Jackalope)
I recently installed Linux Ubuntu 9.04 on a modest 478b socket Pentium 4/512mb RAM Motherboard with out any trouble. The install went amazinly smooth and quick that I was up and runing with Wirless Internet access to my routher with a Belkin Wirless G adapter(F5D7050) in less than 30 minuets. Looks like a great alternative for most uses. http://www.ubuntu.com/GetUbuntu/download