Page Rank is an odd thing. For example,
http://MilitaryHomeCenter.com has a PR of 4, yet it's in the top 3 in Google, Yahoo and MSN for "military relocation directory".
When it comes to link exchange, the only 2 rules should be whether it's in the same genre and if the link page is indexed in the search engines.
MilitaryHomeCenter.com should be in your directory since it is a veterans-only directory of Real Estate Agents, Lendors and Property Managers.