I'm looking for ads to place on users websites, and ads to place on the 404 pages. I don't want to do adsense just in case some people want to host adult related websites. I'm thinking of going the adbrite route so I can allow both mainstream and adult. Should I place 2 links in the header OR 4-5 links in the footer? Also, what should I do for my 404 pages? Any suggestions would be great.