The one page site doesn't really contain useful information, so anyone google-ing "dallas classifieds" that does click the link will bounce pretty immediately.
Do you have any statistics on the number of people that click through the links on your site? Like how many page views has it gotten vs. how many people have clicked on outbound links.
At the same time, "dallasclassifieds.org" has a page rank of 0, so linking it up to your primary site will not really improve the page rank of that site if I understand the basic idea of pagerank correctly.
The only real potential benefit that it can possibly create is that you might get some revenue if you throw some google ads up on the site. As it is right now, you are essentially taking a spot away from a potentially more useful service showing up on the main page.