But just as a note, if u want to only do video encoding/decoding then ur going to need a better graphics card anyway, but why are u getting Vista for that?
Vista is like a mid-end game, it requires the same system performance as a game released around 2003 or maybe a bit later.
So why do u care so much abt Vista and not abt gaming? Its just a prettier version of XP, just like every new game is a prettier version of an older one, with some new features, just like Vista.
There is NO way that card will let u run Vista, because it actually takes away from ur overall system performance when it feels it needs to, and with Vista it will need to all the time (if not more), so getting a new graphics card is a must.