I'm considering building my own Linux-server, using a spare desktop computer. Which distro would you recommend? I'll be, more or less, a Linux-rookie, when starting. Any suggestions?
Will heavy-traffic cause my internet provider to charge me extra (Comcast)? Is it better to just pay for hosting?
I am partial to debian-based distros personally. If you need to start with a GUI (which as a novice you probably do) I'd pick up Linux Mint or Ubuntu (not the server editions, just the normal ones) then follow a guide such a the one posted by Christopher Stewart. If you want to really go all-in, CentOS is a good server platform based on Red Hat. It is better optimized for server use and comes stock with a lot of useful software.
There isn't really a "best" distro for serving though; when it comes down to it it's really a matter of personal preference. Any given distro will let you install all the software and make the adjustments you need to get the most out of it. It's just a matter of what you want to start out with out of the box and which utilities you want to be able to use (for instance, most distros only support one package management system and they frequently come with customized versions of certain software). I suggest downloading a couple different install ISOs and trying them all out over a weekend till you find one that "fits" right.
GUI : good point Justen... once the how-to is done with, it's easy to add the GUI... something like « sudo apt-get install ubuntu-desktop » for instance... but you'll have to check that syntax, it's been a while... ;o)