Asterisk on 64bit

Planning a large enterprise install of Asterisk running with hundreds or even thousands of users.

My question is, is it recommended to install Asterisk on a 64bit or 32bit OS?
Any recommendations on what OS to use? I was thinking CentOS 5.5 would work well.
Any suggestions on what kind of Dell server that should be purchased?

I have one setup with around 15 000 minutes per day, 700-900 users. The server is x64 with 4GB of RAM and 2.3 GHz dual core. Also I’m running an application server there. The load is little :unamused: It can handle double load with no problem.

So stick with CentOS 5.5 x64.

P.S. I like HP servers more.

Thanks for the information.