you have to check if your mobo has an agp or pci-express slot. if it doesn't then you either have to buy a new mobo, or buy a regular pci video card. as far as i know, the fastest regular-pci video card still in production is the nvidia geforce 5700, tho if your system is at least 2 years old it'll probably have an agp slot.
ps: i forgot to answer your question... no, integrated video is NOT upgradable. usually once you put in a proper agp/pci-x/pci video card the mobo will switch itself over to that display and ignore your integrated video.