Your pc will be fine without the display drivers installed. Just uninstall the current drivers following the directions given.
Alternatively, this link http://support.amd.com/us/kbarticles/Pages/GPU57RemoveOldGraphicsDrivers.aspx
gives instructions on how to do it if you are unsure.
Then turn off your system, pull out the power cord from the socket and install the new graphics card. Have you done that before? Just make sure you insert it with firm, equal pressure but without really forcing it. It should pop down into the slot nicely. Just make sure there is space next to the card, on smaller board it will stick out over the next PCI slot.
Connect your video card using the blue vga connector or the white dvi connector depending on your monitor. If you have the option of using the white connector take it, it allows catalyst to scale the image to your monitor more easily in the later versions.
Turn on the PC and you should have a display, it might just be at a very low resolution. Download and install your drivers and that should remedy the situation. Then you are good to go!
As for the bios situation, to my understanding most bios setups will figure it out by themselves, but if you want to check, just keep tapping delete after you turn the PC back on. It will take you to the bios screen. Find the setting there and make sure it is at x16 or auto if that option is available.