Download the new graphics drivers and save them to a place you can easily locate later
Remove the existing graphics drivers
Shut the computer off and unplug it
Open the case
Remove the old graphics card
Install the new graphics card
Plug the computer back in and start it
Once you're back in the OS, (the screen will look strange, because you're running generic drivers and don't have the resulotion set any more) - locate the drivers you downloaded in step 1 and install them.
Not all old graphics cards will be compatible with Windows 7. Have you downloaded and installed the Windows 7 upgrade advisor to determine your computer's hardware and software compatibility? To download this now, please go to the following link: http://tinyurl.com/no4xb7
Also, to better understand the graphics changes in Windows 7 (including upgrade) please review the information available here: http://tinyurl.com/lt29jm
Also, Microsoft does have an official Windows 7 RC Support Forum located here http://tinyurl.com/9fhdl5 . It is supported by product specialists as well as engineers and support teams. You may want to check the threads there for additional support.