This is how I do it!
Adding a video card :
First off, I make sure I have the video card set up disc or download the latest drivers for it.
Uninstall any existing video drivers in Windows. You can go to device manager, rightclick on the display adapter and uninstall.
The BIOS should be set so that the video adapter (display) should be AGP. Change the rate if you need to (you said it was 8x so select that if you can)
Save and Reboot.
The motherboard should make a single beep and hopefully start up into windows.
Once in windows it will try and detect the new hardware. If it asks to look for drivers, click cancel. Use the set up disc or downloaded drivers and install the drivers.
You will probably need to reboot. Once back in Windows, rightclick somewhere on the desktop, choose Properties, then settings and you can change the screen size. if you click on Advanced, you will get more options to change the video details.
I've just had a thought as well. Does the X1650 need a separate power supply? you may have to hook up a power lead to it. This may be causing your problem. I did that with my old GF6800!