Will like to clarify some of the stuff here. I am a linux user for about 7 years (Caldera) and an ONLY linux user for about 4. Already know Mandriva, Debian, Red Hat, OpenSuSE, Knoppix, Backtrack, Gentoo and others. The one am using right now is Ubuntu, first because i like more how debian manages packages and second because Ubuntu enhances a lot every day. Am a KDE user but KDE 4.1.3 i dont really like it. Waiting for 4.2 . Anyway, first, what i will say was tested on a Pentium 3 1000mhz, with 1GB Ram, a Pentium 4 2.4Ghz with 2 GB RAM and a Pentium Core 2 Duo 8500 with 4GB Ram. All with the same adsl 1024Kbit connection.
Running Mangos on, either a console based (Server edition) or gui based (Desktop edition) will give same results for client performance. The only difference you will see, and its obvious, its that having a guid will consume about 300Mbs more. Depending on what stuff you load on the gui, it will grow or shrink 100mb more. For the rest, the network connection, the cpu usage, memory managemente, everything will be 100% ok. You wont have any problems running it in either way.
For the "What linux distro should i use", well since linux is the same as Alternatives, choices, multiple selections, then you need to choose wisely. For example, if you want to start off i always recommend my students Ubuntu, Mandriva or OpenSuSE. Although you get more help for Ubuntu because of its popularity and about all 99% problems you encounter will have a solution. I dont recommend staying on one distro since each one has something different to offer, at least the main stream ones like Red Hat, Slackware and Debian.
So my advice to you would be, type in google the name of the distro, whichever you found the most, that is the one which probably has more help.
Second look for problems that the distro has with your hardware, for example, if you choose ubuntu and you have a nvidia 7200 or something like that type, ubuntu nvidia 7200 bugs or errors. So you know what you are getting youserlf into. You probably wont have any problem whatsoever if you use a pc older than october 2008. and most network devices are "ready for the hud". Hope this helps. You can find me sometimes in the irc channels of ubuntu since that is the main distro in my country.