nVidia recommends a minimum 550W PSU for the GTX 570. I recommend getting a good quality PSU...a small list of quality mfrs: Seasonic, Corsair, XFX, Antec. Please see my signature for links to a PSU guide and a more comprehensive list of recommended PSUs.
Yeah, not sure what you're doing with three pieces of ram, you should have either two or four.
Anyway, Stcxik with PSU's from Corasir, Antec, seasonic, xfx. Get a 650 W PSU just to have some headroom. I like this PSU and have it myself:
CORSAIR Enthusiast Series TX650 V2 650W ATX12V v2.31/ EPS12V v2.92 80 PLUS BRONZE Certified Active PFC High Performance Power Supply http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Because having an odd number of sticks means that your memory will be operating in what is called single channel mode, which is slower than when the memory sticks are in pairs and operating in dual channel mode. There's nothing preventing you from doing it, it's just not recommended. What I suggest, is that you get one additional stick of ram, identical to the other three, and complete your two pair setup.
Sosfm covered it in his first post. If those PSUs aren't available, look for an alternative by Seasonic, Antec, Corsair, XFX, or Enermax/LEPA. Avoid Crappermaster. Even if the stated unit is good, the company has liar-labels on some of their PSUs (fact, not opinion; reviews at HardwareSecrets). That is dishonest, and a dishonest company does not deserve anyone's business, for any of its products.