There should be a label on the back with the shipping specs. Post those if you can so we know what machine you found.
One quick thing to try. Do you have a keyboard plugged in? Is it an Apple keyboard? If it's an Apple one, right after that startup sound, hold and keep down the following keys, Command+Option+P+R. Hold down all 4 keys right after that sound and wait for two more startup sounds. That resets the PRAM.
Since you say the video card is AGP, Is it seated all the way in the AGP socket on the board?
The Startup sound is a good sign as it's probably passing the RAM you installed.
If the keyboard you are using is a Windows keyboard, Hold down: Control+ALT+P+R like above.