Well, the dvd playback will use the video card if you add it, as well as the rendering of the front-end. However the GPS uses pretty all the processor time (because you say in the background).
So it is kind of a toss up. I would think that a new processor would help the most because it can be used for bothe video and general cpu usage, but a lot more costly. The video card is obviously much less, but wont be as effective. Also keep in mind that PCI video cards, really are not being made anymore, so you will be stuck with relatively old choices.