[XFree86] Why does overlay lag behind with XVideo?

[Hakon Gunsen]

Hello!
I have a nvidia gforce2 card, and with mplayer or xine I use the Xv extension output plugin.
But I have noticed that the video overlay lags behind in position as I move around the player application on the desktop. Small problem, but anyways I'd like to know what type of card I should buy to avoid this, so that the video moves along just as fast as when I use the xshm output.

Hakon