But don't stare blind on the numbers, a 650watt enermax would beat a 650 watt MS-Tech three fold. The more expensive brands also have more protections. Like overheating/overvoltage/overcurrent and short circuit protections, which protect youre house from catching on fire .
For a system using a single GeForce GTX 570 graphics card NVIDIA specifies a minimum of a 550 Watt or greater power supply that has a combined +12 Volt continuous current rating of 38 Amps or greater and that has at least two 6-pin PCI Express supplementary power connectors.
For a system using two GeForce GTX 570 graphics cards in 2-way SLI mode NVIDIA specifies a minimum of a 800 Watt or greater power supply that has a combined +12 Volt continuous current rating of 56 Amps or greater and that has at least four 6-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT a crucial factor in power supply selection it is only used as a guideline!!! Total Continuous Amperage Available on the +12V Rail(s) is the most important.
What is your budget range (please specify the currency)?
DO you want a semi-modular or modular wired power supply or is a fully wired power supply acceptable?
What power supply brands do you have available to purchase from?