They are all compatible with your computer - all you really need is a PCI-Express slot and a power connector.
Of course, your processor may hold back the new card sometimes, but you'll still see a large improvement in gaming performance.
And yeah the 6850 is the stronger of the three cards, though the 7770 isn't far behind. You can safely ignore the 550 Ti.
btw.. what do you mean my processor may hold back the new card sometimes??