Just upgraded video card, now my monitor won't work

[BOGGGGG]

To put this shortly, I had a Nvidia 8800 GTS and decided to upgrade to an ATI Radeon HD5870. When I put the HD5870 in the computer and try to turn the computer on my monitor wont recognize it (just goes to standby). However, everything in my tower is running (all the fans, even the HD5870 fan is running). I replugged the 8800 GTS and it worked again.

Computer Specs:
OS: Windows 7
Motherboard: p5n-e SLI
Power Supply: 800 Watts
Graphics Cards: Stated above
Ram: Kingston 4gigs

 

[Leeky]

Did you remove the nVidia software and drivers first? You need to do that with the 8800GTS still installed, then shut down your computer, and install the new GPU, then reboot and download the new drivers for your card for amd.com and install them.

I presume you also have the power connectors fitted to the new GPU when running it as well?

 

[BOGGGGG]

I uninstalled them in add/remove programs AND disabled the 8800 GTS in Device Manager all before putting in the new card. And yes there are two power supple inputs in the graphics card (both 6 pin) and both of them are plugged into the PSU.

 

[blastkannon]

use driver sweeper to uninstall your drivers.