27A on +12V rail.
I think that should be enough.
Just make sure it has two 6-pin power connector for the graphics card.
Minimum power requirement is 450W PSU.
The maximum graphics card power is 150W.
Maximum CPU power is 95W.
The rest of the system consumes power but not that much.
Tell me if it works.
The 550 Ti should be faster than HD 6770.
The 550 Ti requires one 6 pin power connector.
6-pin power connector looks like this- http://kablammo.strongerthandeath.com/wp-content/upload...
You don't have to worry about HDCP support it has been available long long time ago.
3d support and HDCP is different.
If you want support for Phsyx and CUDA you should go with Nvidia.