Dell inspiron 530s need a DVI video card....

just bought a Dell slim inspiron-530s desktop computer and a dell 20 inch lcd monitor. find that the monitor has DVI and VGA connectors and cords to connect to computer, but my computer do not have DVI output. I want to get the digital output quality, that means not just a DVI-VGA adapter, so I think I have to buy another video card which has DVI output, is that right?
which is the cheapest video card could match my computer? anyway, that is not a very good computer.

thank you very much for your response