24" iMac and 2 GB RAM won't boot

My new 24" iMac will no longer start up with the 2 GB of Apple RAM installed(2 x 1GB DIMMs). The Power light indicater on the front comes on, but the computer doesn't actualy start or boot up.
I tried removing one of the 1GB DIMMs and the machine boots up fine with just the 1 GB of RAM installed. Doesn't matter which stick or bank i try, as long as it's only 1 GB, the computer starts up fine. Any idea what's causing this and how it can be fixed?
This problem started today when i tried to start up the machine after it had been shut down and unplugged for 1 week while i was on vacation. Prior to this, the machine ran flawlessly on 2 GB of RAM.
Any ideas?
Greg

Aloha and Welcome to Apple Discussions!
Try resetting the PRAM:
http://docs.info.apple.com/article.html?artnum=2238
Mahalo for the welcome and the advice. I'm now able to boot with the 2 DIMMs installed after resetting PRAM. But, the computer is only showing 1 GB installed instead of 2 GB. Tried running Hardware Test off the CD. It shows only 1 GB RAM, and it and logic board functioning properly.
Any other ideas. Tried calling support, but it's after office hours?

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