I think you are approachng the issue from the wrong angle.
Your problem is:
"my favorite it leaves a bit to be desired (IL-2 Cliffs of Dover)."
Your solution is:
"Well, I would like to improve the board"
So, if you upgrade from a 550ti to a 560ti you will see hardly any performance improvement
If you want to see improvement you have to go to at least a GTX570 2,5Gb VRAM
Better even GTX580 3Gb VRAM
The GTX6xx series are too expensive for now. You need to wait for the new NV Models to come out (Sepember2012) as these will lower the prices of the previous modells.
And all this suggests that trying to avoid changing your power supply will not be the solution.
My 2cents
~S~