Once the DV file is produced (like rendered, with all the bells and whistles), converting it to MPEG is not a video card thing. It depends on the CPU and installed RAM. However if you encode directly to MPEG (which makes sense if you just want to author a DVD and not export to tape or otherwise archive footage) then a faster video card can help (depending on the software you're using; Avid Liquid for instance can take advantage of a better VGA).
Or you may want a real-time dedicated editing card, like Matrox's RTX100.