Mixing RAM sizes and issues

I'm having an issue with my late 2012 iMac 27 inch. It came with 8GB stock RAM installed on the 1st and 3rd slots (lookin from bottom to top). The 2nd and 4th were empty. I bought a Crucial 16GB RAM (http://www.amazon.com/Crucial-PC3-12800-204-Pin-Modules-CT2C8G3S160BM/dp/B008LTB JFW/ref=sr_1_1?ie=UTF8&qid=1365482251&sr=8-1). I installed the new ones in 2nd and 4th slots. Much to my surprise, when I switched on the machine and checked "About my Mac", it only showed 16B 1600Mhz RAM instead of 24GB RAM and after a few seconds, the system hung. I restarted the machine and I heard several beep sounds and the system never turned on. I checked th slots and all were seated OK. On switching again, I heard the same beeps. I switched off the machine completely, waited for a few seconds and tried again and then all was normal. The system was showing 24GB RAM and I was able to use my system for couple of hours and all was OK.

I have couple of questions:

1. What's going on here?. Is it a bad RAM that I need to worry about?. I still have ample time to order a new set of RAM and return the ones I have now.

2. Why the system acted up and now it is OK even though I didn't change anything?

3. Is the seating correct?. What is the best order to get maximum performance?. I intend to remove a 4GB RAM and keep it at 20GB RAM. If that's the case, what should be the order of arrangement?

4. Is there any cause of concern that my system is acting up?. I'm well within my 14 day return period and I can go for a exchange if you guys feel that way.

Please advise.

Thanks.