first of all, make sure EVERYTHING is seated right. Those connectors can be tricky iirc. I used to own a PowerBook 190, and it caused a lot of headaches, mostly from stuff coming unseated. I have also experienced with any machine that has a removable CPU. They have a habit of coming loose. Also, check to make sure the old RAM will boot the machine. Sometimes, if the RAM Is dead, it won't even do the mac equivalent of POSTing.

Did you hear ANYTHING from the speakers? Try plugging in a set of headphones or external speakers. I dunno, but sometimes things don't work right if the screen is unplugged.

Just simple stuff like that.

Also, check the card cage. That was another issue I had. It was annoying since It constantly came loose on me.