I think for Dell systems you have to press the "F1" key upon startup to access BIOS. Once you get to the blue screen, go somewhere into the advanced settings or simply navigate around until you find onboard video. It will let you disable or enable it and in your case, you want to disable it.
Your monitor should support SVGA. Is it a blue color 15 pin graphics port? DVI is newer and better but your monitor has to support it.