I was using a Dell Dimension 8300 2 years ago, and I bought an HIS IceQ HD 4670 AGP card for it, which is still the best AGP card available, if I'm not mistaken. It worked great for DX9 games at medium resolutions and let me keep using the Dell a little longer. As cliffordcooley said, it should see the card after setting BIOS to AGP video, if still no go, try cleaning the AGP slot and video card contacts (after powering down!). Old slots which were unused often accumulate a buildup of dust, and cleaning it or even just re-seating the card a couple times often makes it work. BTW, my 8300 ran ungodly hot with the HD 4670, because of the stupid clamshell case with just one fan for everything, so I cut a 4.5" hole in the side so I could blow a 120mm fan towards the video card. That brought my temps down to acceptable levels. Many others simply ran theirs with the clamshell partially open and a small table fan pointed into it. I also looked into moving the guts into a better case, but the motherboard layout is proprietary and the screw holes won't line up, and the front panel connector is unique to Dell as well. It will, however, accept a standard ATX power supply, so I upgraded to a 650 watt (maybe part of your problem is the OEM 300w PSU?). Anyway, good luck, and keep that old hardware cool whatever it takes.
