Landing Pages, Registrations, and Profile Building After Registration

I've been talking with some of my SEO buddies, and they work for some very large companies. Talking about Landing pages to help generate higher conversions/registrations. One thing that I keep hearing is keep the registration as simple as possible.

I want to modify the registration process to be as simple as possible. ie: Name, Email, Activate. But then once the account has been activated, and the user goes to log back on. I want them to be forced to fill in the rest of the profile information. So at least I have an activated email/user.

I am sure this can be done, but I'm not exactly sure how... This may not be the place to ask, but I figured with all the SEO focused heads here, it might be the best place to ask as well.

You will need a special plugin for this.

Can you recommend anything?

You will need to code it your self or find someone to code it for you. vb.org is probably a good place to go for things like this.