There's no need to upgrade the motherboard, when you have an open x16 slot already (unless the OP is looking to xfire or SLI).
I agree with hang-the-9. A low end 4xxx series card is pretty cheap, low power, and is HD. I would also add, that your PSU may not have a 6 pin PCIe power connector. Before getting a video card, see if it requires one, and check to see if your PSU has one. The video card may come with an adapter, but you'll want to make sure.