atx is a standard so if it says atx it should fit atx coolermaster has some some questionable power supplies i'd look into a seasonic,corsair,antec,enermax for a good power supply but give us your full specifications and we will recommend the right power supply to suit your needs
It's not really about the wattage, it's more about the power on the 12v rail(s). For a single card setup, the 25amp rail on the OCZ is sufficient. And 550w should have no problem powering the newer, power efficient cards.
Of course, in the end, it's your budget and electric bill.