Hmmm, I bet that like Ender said, especially if you are planning a reformat, you could probably just plunk the card into the slot then and go away.
If you wanted to be absolutely, positively certain that the onboard didn't load up, then you could probably disable it in the BIOS (if that's your thing).
I know my motherboard has onboard video, but I always put in a video card from the get go and it never complains to me :D