I would suggest the PC Power & Cooling Silencer 610w, instead of the SeaSonic, if you're looking to spend in that price range.
Newegg.com - PC Power & Cooling Silencer 610 EPS12V EPS12V 610W Continuous @ 40°C Power Supply 100 - 240 V UL, cUL, CE, CB, TUV - Retail
But the Corsair 520HX will last you a long time too. Will easily power an Agena and an extra GT.