I would park the additional domains for now.
Start building content on your new forum and blogs first. Once you have some content on the sites, try to find some other forums and blogs related to your market and start participating. This will allow you to start building some credibility while also getting some ideas for more content on your site.
See if there are any online directories that are related to your site and try to get listed. DOn't pay to get listed with spam type directories or link exchanges, but if something looks legitimate, try to get listed.
Have you done your keyword research? Use a free tool such as Keyword Discovery or the keyword tool within Google AdWords to see how many people are searching for your keywords.
Since this is a very focused market, make sure you know where people interested in this subject gather. Go to those places and create a presence for yourself.
If all elese fails, or if this is taking too long, try doing some pay per click advertising. Make sure you have some stat reporting tools on your site so you know where your traffic is coming from and what they are doing on your site. Keep an eye on your stats and keep tweaking your site until you start to get more people exploring the site and signing up.
Good luck!