Without knowing which iMac you have I can't give a specific answer. (Go to your Apple menu, About This Mac, More Info, and get the specific model name.) If it is an iMac 7,x, that could be the problem. In the Apple Forum there are a number of strings that are experiencing the same problem with upgraded RAM. Actually, I have the same problem and will need to try and have my cards replaced. 

Basically, the problem is, many iMac's will not read cards that differ from their specifications. For example, if the manual or data specifies 2 - 667MHz RAM cards are to be used, and you replace the original with 2 - 800MHz RAM cards, you get the blank/no start screen. While the iMac will recognize 1 - 667MHz card and 1 - 800MHz card combo and start right up, it refuses to recognize 2 - 800MHz cards. Don't know why, but they are just picky that way.

Solution (if this is the problem) double check your manuel, and/or go through the above steps to find out the SPECIFIC MODEL of your iMac, then see if you have the correct RAM in hand, if not you will need to throw yourself on the mercy of Crucial for replacement cards...... just as I am going to have to do to get my iMac to work with 4GB of RAM.

Hope this helps