I currently have a Windows web server (2000 standard) and am upgrading it. Before I choose an OS (enterprise vs standard) I would like to know what is ideal for future options to have multiple servers for web traffic. Do I need to invest in enterprise to do this?

Not exactly related, but my first suggestion would be to split the database traffic, and then the static images, from the first server. That way you split the load without having to get fancy with clustering.

Another suggestion I heard recently for increasing your performance is to split your static graphics up between different virtual server names - such as image1.domain.com image2.domain.com etc - but physically only on one server.

A web browser by default will only open 4 connections per server, so if you have a lot of graphics, you will get a performance increase by faking multiple servers.