What's your price range?
4 gigs of RAM and 400 gigs of HDD space on a server rig are going to cost you a pretty penny, and a Red Hat Linux license isn't going to be small change either. Do you absolutely need those specs or are you willing to settle for server-class hardware in a lower price range?
I guess if you're looking for used machines, eBay's not a bad place to start. There's also a place called
Intechra Outlet (merged with the former
RetroBox) that offers used/referbished computers, including servers. A lot of their inventory might be older and more out of date than what you're looking for though, and may be nearing its end of life.
Another option is to buy new. I'm quite partial to the
IBM System X series, particularly their
Tower servers. As a bonus, I think on most or maybe all of them you can get Red Hat pre-installed. If IBM isn't your cup of tea, then I know Dell, Gateway, HP, and lots of others offer a wide array of servers too, though I have little experience with those particular lines outside of the HP ProLiant series.
Finally, about the OS -- if you're just trying to get familiar with Linux or Red Hat, but don't necessarily NEED the Red Hat specific features such as up2date or Red Hat Network, then
CentOS is a very high quality replacement IMHO. It's mostly Red Hat minus the trademarks, artwork, and some Red Hat proprietary stuff. I use CentOS extensively and have found it to be very stable and an excellent server OS. Plus it's got a great community behind it and the price isn't bad either: $0
Hope that helps!
