Do you have a VGA cable? I would hook up the VGA cable and see if a signal to monitor showed up. In fact, I would leave the DVI cable connected when you run the VGA cable, leave both cables connected. It sounds like the 260 GTX is powering up, but there is no signal to the monitor. Try VGA setting on the monitor. Maybe the monitor input selection is the problem. Also with the 6800, you might try a different (lesser) screen resolution setting, shut down and install the 260GTX. Also, which driver are you using for the 6800? If you have a driver for that card installed, uninstall it before you install the 260GTX. Just my thoughts.
The only information I could find on the power supply is that it's a Best Buy product. I wouldn't buy anything that's critically important to my pc from Best Buy. I couldn't find any specs on it either. I'd have to assume it's Thermaltakes "cheap" line of power supplies.
that doesn't nessessarily mean it isn't any good but ..............
You may try booting into windows with the 6800, then install the 260GTX video driver from the installation CD (this driver is tested and known to work properly). Then shut down and install the 260GTX and boot up into the newly installed 260GTx driver.
Well Badge, took your idea of lowering the rez and installing the drivers first.
Reboot with the new card and low and behold I have an active video card.
Everything is working as advertised. So thank you. I am still going to look into the power supply. Thanks again.