How do I prepare my iMac for sale? And best practice for transferring media to a new system.

This is my first time posting in a forum, so I will apologize in advance if I'm doing something wrong. I've tried to search for a very clear answer to my questions in the existing discussions, but have not had much luck.

I ordered a Macbook Pro today and would like to sell my current iMac to supplement some of the cost. I purchased my iMac in March of 2011 with OS X Snow Leopard preinstalled. I've since updated to Mountain Lion. When my computer came in it did not have OS DVDs, so I am not sure exactly how to take it back to factory default and remove ALL of my information safely in order to prepare it for sale. I'm afraid to tell it to format if I am going to need a boot disc/drive to reinstall the OS. I've been trying to research all day and have learned that I will not be able to sell it with the current OS, only the one that came with the computer at the time of purchase (to the best of my understanding).

Secondary to all of this, I have a question about backing up info and porting it to my new system. I have run the Time Machine backup to my external hard drive. However, I DO NOT want to port it to my new MBP as an image of my past machine. I did that when I got my iMac from my old MBP and my iMac thinks it is a MBP (MBP EFI/SMC Firmware Update Icons in my system utilities folder as well as identified as a Macbook Pro on my network). I've never been able to get all of these things to go away and I feel like it's chewing up hard drive space for no purpose whatsoever. I also worry that it could create system issues that I'm just not savy enough to catch before they are beyond control. That being said, I'd like to begin fresh with my new MBP. What is the best method of bringing my photos from iPhoto and music/movies/books/etc from iTunes to my new system? My word documents are easy enough...I keep 3 flash drives updated with them at all times.

Thanks in advance!