is it good to buy and redirect 301 domain names for SEO purposes?

I found a directory with bunch of expired domain names – the directory page where those domain names are actually has PR = 3

my plan is to redirect them to my main domain

Is it good for SEO or matters little?

Not if the search engines catch you. Than bad things will happen to your search ranking.

In my experience, It’s much more effective to simply blog (v6transition.com) about your site and to have a facebook (facebook.com) account for your site.