OpenBSD. I think it's one of the best OS's as far as stability come's and it is compatible (as far as I know) with the spec's you provided. You should experiment with whatever you'd want to, but my suggestion is OpenBSD or perhap's SuSe? Anyway's, ya always have AO as a resource for help on whatever you choose so you'll learn regardless.
Btw: Just being a dick, but this is in the wrong forum