I am building a moderate budget gaming computer that I plan on overclocking later on. The build is an Intel i5 4690K, ASUS Z97-A motherboard, a Samsung 840 Evo SSD, and some generic 1TB HDD. The build is sort of an evolving build as I will be adding parts like graphics cards and ram slowly. I will run an older Nivida graphics card to start but I will get an AMD r9 280x as quickly as I can afford it. Eventually I plan on running dual graphics cards. My question is what is a decent but affordable power supply that will let me achieve running my small graphics card right away but allow me to upgrade to dual r9s and OC later on?