What linux OS would you recommend. And other things.
 

My site's looking to move to a new server, it's a P4 3.4 GHZ 2 gigs of ram and 2x180 gig HD's.

The admin who handles all server functions is thinking about putting Ubuntu on it to run the site.

We're also using PHP5 and I don't think our server is performing as well as it should.

I haven't done a server optomization yet at vb.com but might after I get some recommendations on how to setup the new server.

Ubuntu is pretty nice. But it's more desktop oriented, even if it can be effectively used as a server. Personally I use FreeBSD or Gentoo for servers.

Our servers use Fedora. Seems to work fine.

If it's Linux, why not use CentOS. Basically a free version of Red Hat Linux. :)

How many users would this type of server configured correctly be capable of holding?

It would only be used to host one forum, and possibly a small front page script.

well depends what you like... easy? go with redhat/fedora or something.
configure everything? gentoo, maybe debian
all the latest software & bugfixes? gentoo

afaik red hat is still most widely used as server software
personally i like debian... It's like gentoo, but without compiling every single software pacakage you need...

i can't say anything about user numbers or anything because that depends on too much things, hardware (combination), os, software optimization etc

I have a mix of OS in my servers, have Fedora Core 2, FreeBSD, Red Hat Enterprise and CentOS, the best are, FreeBSD and Red hat enterprise

The reason I love Gentoo/FreeBSD is that I compile almost every software package I need. This means I can automatically put flags into the compiler to optimize it to my system, thus resulting in an overall faster server. I do like Debian-based distros too, especially Ubuntu. I can't say much about RHEL (I haven't used often), or CentOS. I use Fedora Core 1 on one of my servers and it performs okay, but from what I've read the later versions didn't do so well, however I'm not sure about that.

Linux?

Fedora or CentOS.

Fedora if you like having the most up-to-date things.

CentOS if you don't mind using older packages and other things, as long as they have proven reliability.

For those recommending FreeBSD and Debian, these are not Linux. They are Unix-related, or *nix.

Linux stands for "Linux is not Unix" ;)