Before purchasing a new card, make sure that it will fit in your case, and that you have a PCIe slot on the motherboard (you should). You also need to take into account your power supply wattage.

Take note of your other PC components, such as your CPU.. don't buy an expensive card just to find out that your CPU is bottlenecking your system.

To turn off your integrated card you can do what Jibe recommended, or turn it off in your BIOS. You may find that you need to use the bios to make sure it doesn't try to re-install itself.