Like Marin says, XP might limit you - I know my PC was limited to 2gb, although could run 3 but the FSB (board speed) would be limited... that was the board, not XP.
4GB should be OK with XP - I'm a bit rusty now, but google XP memory limit...think you'll be ok
Graphics card won't be a massive issue for DVD authoring an processing data etc....it's raw processing power - so look at AV forums to see what's the best. The fastest graphics card won't help that, although it will be great for the latest games .....