Since putting affiliate links on the homepage of my site, it seems to have dropped off the 1st page of Google. Do out bound links negatively effect ranking?

Hi, I'm wondering what's the best character to use as a spacer in a URL? ie. www.mydomain.com/this-is-my-url.html www.mydomain.com/this+is+my+url.html www.mydomain.com/this_is …