On some, yes - on others, maybe not.
Some boards will also have an AGP slot, which can have a garphics card fitted. Some will have PCIe (PCI express) slots, which can have a PCIe graphics card fitted.
For those which only have PCI slots, it's often not worth upgrading to a seperate graphics card, as there is only a small selection, and with modern motherboards the integrated graphics are good enough not to warrant fitting one. In these cases, more RAM is often a better bet, to make up for that used by the integrated graphics.
So - maybe yes, depending on what the system you buy is.