Catch 22 im afraid,if you have onboard graphics this should auto enable once the graphics card has been removed,however not always,you need to go into the Bios and enable the onboard graphics.....so therefore you could be stumped either way.
Hmmm, bit pessimistic. I thought all mobo's had on-board graphics otherwise you wouldn't be able to set them up "out of the box"! I have rebuilt several computers and removed/installed several VGA cards and NEVER had to go into the BIOS to enable anything ! Maybe just lucky eh! But to answer Tinkey Winkey's question - "No if you leave the u/s card in" ;-)
PS Tinkey Winkey - look at the back of the PC and see if there is another blue monitor connector, usually vertical and not far from the mouse/keyboard sockets. This will tell you if you have mobo graphics or not.