Is A Domain Name Actually Important?

Hello, everybody!

This is my first post... what a great website, and an incredible wealth of information.

I am a newbie here, and I am finally starting to catch on to the basics of SEO and AdSense. My question is this:

For the sake of SEO and appearing high on search engine rankings, is a domain name actually necessary? I have a .com website that is a collection of different articles about various health related topics. Each different section is basically a different website, but is there a significant difference between, for example:

http://www.thisisjustanexample.com/depression
and then, let's say, a different section for drug addiction:
http://www.thisisjustanexample.com/drug_addiction

versus

http://www.depressionsolutionsforeverbody.info
http://www.drugaddictionsolutionsforeverybody.info


Hopefully my question makes sense -- what I'm trying to say is, do I need a separate domain name for each and every topic?

Thanks for your advice!

I always do better with a domain name that contains my niche keywords or relative keywords.

Example:

medical.com/depression
-is better than-
racingcars.com/depression

Not saying that a site with good content with a weird name can't be successful, its just one of many ways to help your site get good targeted traffic.

Good Luck!