No signal on monitor when hooked up to Radeon HD 5750

I've had a series of issues getting a recently purchased HP P6510Y (with Windows 7 64-bit) to work with a Radeon HD 5750 graphics card. I bought a new power supply to compensate for the upgraded video card, but the power supply appeared to be a dud (I was getting power to the motherboard, but the computer would not turn on). The computer worked fine with the original power supply, but not with the new one.

I returned that one and got a different one from a friend that he had been using for about a month, so it definitely works. However, we ran into the same problem.

To attempt to remedy this with minimal headaches, I have the original power supply powering everything but the video card, with the video card using the new power supply (Coolmax CL-700B, which should be more than enough to power the video card). The video card is getting power and the fan on it works fine.

When I hook the monitor (HP 2010i) up to the integrated VGA port it works fine. When I hook it up to the video card's VGA port, it just says "No signal" and the monitor goes to sleep.

I've tried to uninstall the old drivers and install the new drivers (though the ATI Catalyst doesn't appear to be any different). The ATI Catalyst still only recognizes the RS880 display option, which is the integrated video card. I have tried unsuccessfully to have the computer recognize that new hardware is installed. I confirmed that the video card is installed properly.

Short of installing this video card on another PC to check if it works that way, which is not really possible at this point, I don't know what else to try. I think I've given all the relevant information, though if there's anything else I can I'm open to any suggestions. I'm going kind of nuts with this problem and I have a copy of the Final Fantasy XIV Collector's Edition on the shelf next to me taunting me. Any help is greatly appreciated.