A 500W PSU would be enough for a single card.
Might as well take the Extreme3, as I think it's the cheapest. You don't need a fancy mobo. Most people'll get an expensive board either for features, like tons of SATA ports, or better CPU overclocking. Most people don't need the features, and you won't need to push the limits of your 2500K (4.0ghz would be just fine.)