After install you would want to download the latest drivers online directly from Gigabyte's site for your board, or the source of the driver such as Intel, Realtek, ect. I could link you to the latest drivers, but you didn't say what motherboard you have, so here we are
DVD drivers will work, but they are usually out dated by the time you even buy the board when it;s first released, so it's always suggested to get updated ones online when installing.
Yes, if you install from the CD, use the manual method and choose ONLY the drivers you want to install. DO NOT use express install.
Order of installation is not important, and the drivers will update anything windows installed if there is something new in the drivers you are installing.
BIOS is not affected by the operating system, so the settings will always remain the same unless you clear the CMOS, change BIOS settings, or reset the BIOS to defaults.
Hope that helps! If you need the latest drivers, please tell us what motherboard you are using, graphics card too if you want the latest drivers for that.