why desktop effects needs so much graphic acceleration, almost all of my card?
Well, if you have things like "window transparency" or "Compiz Effects" turned on, these are supposed
to take some extra Video Card resources. After all, when you're not doing something 3D-intensive (like gaming) with a powerful card, why not use all the horsepower to render the desktop?
However, if you don't want to tax your Video Card anymore than necessary, you could just turn off those effects (and maybe run a light-weight Windows Manager, like fluxbox)