For a system using a single GeForce GTX 550 Ti graphics card NVIDIA specifies a minimum of a 400 Watt or greater power supply that has a combined +12 Volt continuous current rating of 24 Amps or greater and that has at least one 6-pin PCI Express supplementary power connector.
For a system using two GeForce GTX 550 Ti graphics cards in 2-way SLI mode NVIDIA specifies a minimum of a 500 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.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Continuous...
For a system using a single GeForce GTX 550 Ti graphics card NVIDIA specifies a minimum of a 400 Watt or greater power supply that has a combined +12 Volt continuous current rating of 24 Amps or greater and that has at least one 6-pin PCI Express supplementary power connector.
For a system using two GeForce GTX 550 Ti graphics cards in 2-way SLI mode NVIDIA specifies a minimum of a 500 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.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Continuous Amperage Available on the +12V Rail(s) is the most important factor.
I've never heard of a bmg 550 power supply unit but I've heard of BFG power supplies.
Please post the correct brand and model of the power supply unit so an accurate response can be provided.