OWA Same Machine as Exchange or not?

I am running Windows Server 2000 and Exchange 2000. I have set up OWA as a test and am using it on the same machines as Exchange 2000. I seem to be getting conflicting or no information to the question do I need to be running OQA and Exchange on different servers and why? Can anyone give me the reasons pro and con why the same machine or different. Or point me to a reliable setup guide for OWA.

Thanks