upgrade

I am in the process of switching to exchange 2000. I have a mail server and another exchange 5.5 eserver that is used as a SMTP gateway. First is it better to build new or upgrade the 5.5 to 2000, and second what or how should I deal with the SMTP gateway exchange server. The gateway deals with the internet and the mail server is for internal.Any suggestions on the best way or any articles of someone else with the same problem.