This is my first pc build. I'm still learning the ropes and with my current part choices, will I need to purchase any supplementary cables besides the 6 pin sata power splitter I've already purchased? http://pcpartpicker.com/p/WQqPQ7
No you don't. For the GPU you need a 8-pin and a 6-pin connector the PSU you have only has 2 6-pin connectors, so you should upgrade to at least a 700W PSU to ensure you have enough power and connectors for the 290.