futureX - 4:10 pm on Aug 3, 2010 (gmt 0)
As far as I understood it you don't generally gain any boost to your own site for linking out, only links in count. Otherwise you could build a dodgy site and just link to all the best sites to get ranking...
That said however it may increase your exposure in Google in a few ways:
Firstly if you are linking to sites that have content relevant to yours and including around 750 words to describe it you're going to create some keyword rich pages which Google may hit, the downside being that many of those may just follow the link to the linked site.
When carefully planned it could boost the keyword relevancy of your whole site, being that every page with information about these sites will presumably link back to your main site or pages then it could be beneficial.
These however are only fringe benefits of creating the directory. You would gain more benefits if you spent your time writing relevant and interesting articles with your time. The only way I could see a directory being relevant to Google is that the sites you're linking to might notice some hits coming your way and offer you a link in return.
If you want a directory and you think it is relevant to your site then go ahead, as long as you don't link to bad/irrelevant/blacklisted sites then it will not do any harm.