Know-How Exchange : SEO/SEM

Yes.

It's sort of a chicken-and-egg problem. You build a contact list because people have opted in to your communications. They opt-in because they believe you have information that they want.

But first, you need to get them to your website, which means that somehow your target audience needs to find you when they start looking for you. So you also need to invest in bringing (anonymous) people to your website, to entice them with your offering, and want them to continue talking with them.

Also - studies show that the more information you ask of people to opt-in, the lower the sign-up rate. So, only ask for information that you absolutely need to have to begin the dialog. Once you've show your true worth, they'll share more of themselves with you.

karen and Jay are right. I'd follow their advice about the user experience. Perhaps your question was more basic than that.
To answer the simple question: Yes, you can create a database of prospects by having them sign up for something.
Here's a simple plan:
1- Answer the question "Who is my target prospect?"
2- Answer the question "What problem does my target prospect want to solve?"
3- Create something of value that will help your target prospect solve the problem. For instance, if they need a local computer supplier you could create a white paper called "How to Choose a Local Computer Supplier."
4- Offer the thing of value, in exchange for their email address. This could be done with a simple Google ad:
Download Free Whitepaper
"How to Choose a Local
Computer Supplier"
<website url>
5- Create a page on your site (the landing page for the ad) that tells a little about the thing of value you're offering, and allows them to get it by giving you their email address.
Voila! email addresses to put in your database.
The key to this is to make it a truly valuable thing, not just a sales pitch for your own company. If the people feel like their time was wasted, they won't be receptive to any future marketing you do.