We have 1 Big Butt site that converts good and retain that would make a great addition to what you have.
http://www.topbucks.com
http://www.bubblebuttsgalore.com
May not be exactly what you're looking for, but it's close and I'm sure that it would work well.
I'm heading to bed since it's 3am, but if you have any questions hit me up on ICQ and I'll get back to you when I get up. (assuming that you contact me haha)