As far as choosing redhat for that computer, I would say that is not a good choice. Slackware would be my choice for you. Many others would also suggest debian. Redhat needs more comuting power than that PC can give it. Mandrake is the same way. There's not a site where people can get everything you need to know. Slackware has a good install guide on their site. Ususally you can find almost all the information you need from the site of the server product you are using (ie if you are setting up sendmail, then www.sendmail.org
would provide you with plenty of info) and by search here and google.
Here is a google link with some sites that have many command line commands:
and to answer your last question you can look at webmin for remote administration with a gui. www.webmin.com