I wouldn't be wasting money on an Athlon 64 if it's not a gaming machine, but that's me.
It doesn't really matter about the videocard. The Radeon X700 isn't a bad deal, but it's still overkill for what you're doing. The X550 will be fine. Any new videocard will play TV episodes easily, and since that's all you're doing, there's no reason to buy better than that!
-Lasereth