More memory or a faster card will do nothing for editing or a DVD conversion.
The work you are talking about is done on the CPU and platform, not the video card.
Even an X1300 might be overkill for you. You can really get the cheapest card you can find if you don't play any 3d games, just make sure it isn't a 'Turbocache' or 'Hypermemory' card... those will bleed memory from your main RAM.