Wanna build a Ubuntu Server

I want to make a old PC to be a server,
not for desktop, I saw this.

http://www.ubuntu.com/server

Ubuntu is good for desktop, but how about server?

Another option you might want to consider is Gentoo. Not sure how it compares with Ubantu, though.

http://www.gentoo.org/

The link below gives some info about setting up a Gentoo server.

http://www.mydreamrpg.com/community/showthread.php?t=122

Probably fine. However if you want a server solution, you're probably better off going with a real Debian rather than ubuntu.

What's the most widely used Linux for Server? Debian?

Debian is the way to go. I converted to debian a few months back and haven't regretted it!

3 Servers running Debian
1 server still running Win2000 Advanced Server (soon to be retired)

Red Hat Enterprise Linux. But Debian is one of the best, and it is free (as in beer).

I use an old computer with Ubuntu as my little test server.
I'm not using the server edition for that one, though. Just regular desktop with the appropriate packages.

It works well, but other distros which have been around for longer may be more suited since they've had more time to sort out any security issues.
Ubuntu is certainly worth a try, though!

It seems my old machine got new life with Ubuntu... it's really fast.

Since it serves as server I don't install unnecessary packages, and it works nice with pre-configured L.A.M.P