The 550W that you have is adequate.
The card doesn't take 310ish watts. I think you are confusing full system load with just the graphics card load.
A HD 7950 peaks at 188W under full GPP load, less under full rendering load. (Power Consumption : Radeon HD 7950 3 GB: Six Cards, Benchmarked And Reviewed
The CPU you have is pretty decent effeciency at about 100W tops, then the MoBo, RAM, HDD etc is probably another ~75W maximum.
So, that's more or less 365W using rough, safe, numbers - getting close to the 310W you say you researched, I reckon you are getting mixed up with system load.
Although a power supply is important, a general rule of thumb is calculate what you need (say the 365 above) then add another half for over provisioning. so we're talking about 548 - aka 550W. Hence that 550W OCZ PSU you have will handle that load very comfortably! No need for another PSU! Quality is more important than big wattage numbers when you have an adequate PSU - and the OCZ is quality enough. Don't waste your money - it's something you can consider in future when/if you go Crossfire or what-not.
Think of it this way, your system under full load (rarely achieved) will max at just 66% of the PSU rating! And this is using rough, on the safe side, numbers.