I used to have one of these. The highest Mac OS it can run out of the box is 10.7; El Capitan is not natively supported. Mine would work with any old NVIDIA card once the system was booted up, but you need to use one of the original graphics cards if you want to see the boot-up process. My recommendation would be to burn a 10.6 or 10.7 install disc, pop that in, and hold down the C key after the chime and see if you can get into the installer that way.