you're right, CentOS is RH based, but you know, RH, Fedora, essentially the same thing.
that said, livecds, that depends. Not all distros offer live cds, and in many cases just because something runs on a livecd doesn't mean it will run when installed. I speak from experience.
On my laptop i ran the Mepis live cd perfectly, but after the install things started to go VERY WRONG, VERY FAST! On the flip side, the Ubuntu livecd crashes and burns on my laptop but I am using an installed version right now without any problems, in fact it has the best hardware support for my laptop of ANY DISTRO out there. Similarly on my desktop the Ubuntu live cd runs fine and dandy but when you install it, hell, it freezes on start up (i assume its a xorg issue but i am not sure).
Same could be said about Kanotix running in livecd on my lappy but installing pretty poorly.
That said, livecds are a hit or miss, so are installs. your best bet is just to try a few diff distros and see what you like best. I would set up different types too so you have a general spectrum of things.
here are a few suggetions.
by the way, try gnome and kde, as well as some of the lighter guis to see which one you like best. i prefer gnome myself, but some people can't stand it, so try the VMs and see which one you like best for yourself.
ok, now for the distros:
Category : Name : Why
Debian spin-off: Ubuntu/Kubuntu/Xubuntu: Has a huge user base (ubuntuforums.org) and fantastic hardware support generally. It also gives you the power of Debian's apt-get which is in my opinion the best way to get software, PERIOD!
Source Based: Gentoo: What can I say, the choices are few. if i were you i'd go either Onebase or Gentoo. Gentoo will be faster than onebase, and it does have a larger software library and a forum that is always filled with good ideas.
RPM Packaged : Mandrake 2006 : Better hardare support than Fedora, and hell, 2006 is FAST. yes older Mandrakes were pretty slow and urpmi does suck (compared to apt-get) but its pretty fast at least in terms of boot time.
Slack-based: Vector : Based on rock-solid slackware, but fast as a mo-fo.
for mroe ideas visit distrowatch.com