Domains + Directory?

[iPlox]

I have a reasonable successful domain registration business at iPlox.com. I was considering installing a directory on a sub-domain on the website. However, I am new to directories and have never managed one before. Considering I can easily get my directory up to PR3 or 4, how successful can it be and what kind of revenue do these generate.

Also, is it a good idea to have a web directory along with a domain registration business?