|"which card should i buy ati |or intel or nvidia"
That's completely up to you. I almost always use nvidia cards in my computers because they're stable, well-supported, and have no problem reaching extremely high clock speeds.
But you can't go wrong with a power efficient ATI. ATI is currently the only GPU maker to have a complete line of DirectX 11 graphics cards available, and their DX11 HD 5870 is currently the fastest single-GPU card out there.
First of all, how much are you willing to spend on a video card?
Is your power supply capable of satisfying the demands of a mid- to high-level video card (take a look at the sticker on the PS)?
EDIT: And no, the onboard graphics won't be an issue. It can be disabled in the BIOS.
Now fully Se7ened.