It really depends... with long runs, you'll usually get some jitter or ghosting or whatnot, but it also depends a lot on the quality of cable you get. Be sure the cable is one that has the three coaxial cables inside (for RGB) and not just cheapy twisted pair or something. It should be a thick, stiff cable.
Look at pccables.com, for example. They have the triple-coax VGA cables, all the way to 100 feet.
tooki