Dual Xeon necessary???

I will soon be purchasing an HP ML350 server with a Intel Xeon 3ghz processor. I have the option to upgrade and add another Xeon 3ghz to the motherboard, but I'm unsure if it's necessary.
I will be running Windows Server 2003 enterprise with ~50-80 users concurrently logging in to the server.
This will be the only server managing users on the network. It will also run a couple of other programs (maybe), like, Norton Antivirus, microsoft SQL server.
It will be our DNS, WINS, and DHCP server. It will also be a file server for the user's home folder.
I can't think of anything else off the top of my head. Basically I would like to know if I get a second processor will it be utilized effectively.
Or does windows even share the load if it has dual processors?? I know they didn't in the past.
Thanks

If you are running SQL server get dual CPU's.
Have you factored in any high availability or distaster recovery?
If not rather than having a single dual CPU box get two single core
systems and make sure you can mirror them in some way (via cluster server or some sort of nightly clone).
50~80 users not being able to do anything cos the server is down
can be very costly