I'll say upfront that i'm not really a fan of 2nd Tiered Networks. The reason for this is I've ever tested up to $600 on 2 different lower tiered PPC networks and the traffic from these sources are not high converting.
The second of traffic that you want to be careful of are Banner traffic. This is CPM which means you're charged per 1000 impressions of your banner shown. If you want your banner ad to be shown on top of others, you will need to pay more per 1000 impression. So lets say I decide to pay $10 while you decide to pay $5 per 1K worth of impression, my banners will show up in place of yours.
I don't recommend banner advertising unless you're good at tracking your results from this source. One way to do this is to create a campaign that is solely targeted at a certain site. Have a conversion tracking code on the thank you page, and watch your conversions. If your ad spend is less then your earnings, then keep the banner running, otherwise, take it down. You'll also want to be careful of where your banner appears. This is because some banners are placed in non-ideal positions and even though the page loads, your banner will not be visible to the websites visitors.
So be sure to check the site before going further with the publisher.