Should be fine. I can't imagine any layout that would not allow for that card to be dropped in. Most of those cards take up two slots, so make sure there's not another card in the slot directly below the PCIe x16 slot.
BTW if you can afford it, bump up to an HD 7750. You'll see at least a 50% performance bump and the card actually draws less power at the same size as an HD 6670. http://www.newegg.com/Product/Product.aspx?Item=N82E16814161403