Your graphics card will have the DVI outlet connection on the rear panel. Usually you find both the VGA d-sub and DVI sockets if DVI supported. As your PC is three years old I doubt it. That isn't to say tho' that you cannot purchase a new graphics card which does. There was a posting on the forum some time ago now from the FE when he purchased a new 19" AOC TFT with both analog and DVI support. He said that he noted little difference between using analog and DVI.
I have a 19" Samsung TFT with support for both systems but only use the analog connection because my graphics card only supports two analog outputs.