Most modern boards with onboard graphics will take a new card with out the need to alter the bios. As you have found out, there is no setting to disable the onboard vga.
Switch the computer off, insert the new graphics card and boot with the driver CD in the drive. It should auto detect it and fire up. Oh and connect the monitor to the new graphics card. lol
Yes a 4X board will work in a 8X slot.
Enjoy the extra memory:-)