But i would reccomend calling a computer tehnician before doing that.
I agree! You would not want to accidentally disable something in the BIOS and have your computer not boot up at all
Not sure if your computer was custom built, but if you can find the manual of your motherboard, there is usually information on where the onboard GPU option is and how to disable it.
Don't worry about uninstalling the intel hd graphics if you ordered a pci-e/agp card. I rather keep them in case the new gpu card breaks again and you need access to your computer. Without the drivers, everything would look big and difficulty to browse the desktop.
As testmachine said about disabling the onboard gpu, I would do it. It uses your desktop memory (aka shared memory) and the fact your have a gpu card, the onboard wont be needed at all, unless you have problems with the new card.
as for finding drivers, listen to the king of emulator-zone
My suggestion would be to get the latest drivers from the website, they probably have some needed bug fixes compared to the ones in the install disk.