Adding your web host to web host directories help a lot when you want to get the word on there. There is no doubt about that.

Some directories charge a fee to add your website to the list (which I avoid doing) There are other proactive ways to spread the word than throw money at web host directory.

As mentioned many big hosts like Gator/Lunar etc dominate web host directories and are mentioned everywhere when some one searches for a web host - we cannot escape that.

Try find a "niche" - a good selling point that separates you from the rest.
Bear in mind that you need to target the right audience as well.

Press releases are also a nice way to get the word out there.

Good luck