Yes, you can always disable the on-board (usu. "Features") display device.
BE SURE TO FIND OUT WHAT KIND OF SUPPORT YOU HAVE for an add-in graphics card BEFORE you go out shopping for one, tho! (Possible types are: PCI(obs.), AGP(obs.), PCI-E (x1-x16, higher == faster).)
While you CAN find one of the obsolete ("obs.") types - even for under $50 - they're pretty old at this point, and might NOT be worth using as an "upgrade" from an on-board chipset...
If you post your Mobo brand & model I can find out what it supports, ans what graphics chipset it has....