Is it better to have the main site, sales, support, and everything for a company under one site with one userbase and template etc... or is it better to have individual departments, like different sites with different templates and a different database?

depends on their connection. will you be presenting them as a singular entity (all under the same name) or will they be separate yet owned by a parent company.

for example, if you ran a hosting business but wanted to expand into web design, it might be a good idea to separate them (take 2advanced.com/.net ). however, if you are talking about many pages all relating to the same business, its much better to have them all in one site. its a lot better to have 10x the content on 1 site than 10 sites with 1/10 the content. have it all in one place for your visitors.