Unless I'm mistaken you won't have to remove anything. There is no card installed, it's a chip on your motherboard, i.e. integrated graphics.
Installing a new graphics card should be pretty straight forward.
The Nvidia 6X series card would be a good choice although an ATI X8 series might be a better choice. They offer similar performance with a slight edge to the ATI cards, at least according to this site. The Nvidia cards do have the advantage of supporting HDR 3.0, how beneficial that is is open for debate. Either card can be had for about the same price.