Hmm... try putting just the old RAM in, does it boot then?
if so, then its something to do with the new chips that it doesn't like. Are you sure they are the same speed?
But if it doesn't work still, then somethings unplugged or been made loose so just push those cables in further on anything you can find to make sure
EDIT: Nuke...now that is stupid
...I mean, as if anyone will have a sledgehammer lying around in the house...