I got my old PCI card and installed it, with the same problem. Does this mean it's not necaessarily an AGP/PCI slot issue? Everyone else I talk to insists it's something to do with it failing the POST check, and most likely something to do with my ram or the new heatsink I installed.
I called up ASUS's tech support, and he said the long beep means that it's not passing the POST check, which COULD mean a faulty AGP slot, but more likely because of something I did to my ram or CPU when I installed the new heatsink (like you guys suggested). It could also be a voltage issue. He said that if you use too much thermal grease, it can act as an inductor (or conductor? I don't know my electronic terms too well), and that may be interfering with something on the motherboard. He suggested removing the thermal grease with some rubbing alcohol, and try all of my compnents (such as videocard, ram, etc.) on my buddies computer just to make sure they all still work.
Any suggestions/ideas?