The 5770 should be dropped from consideration altogether, it's an old piece of hardware. The 7750 or the 6790 will be your best bet. They are both more powerful than the 550 Ti.
The 6790 is a little more powerful than the 7750, but the 7750 requires no extra power connectors. Just plug in and go.
Don't expect too much from a new graphics card though. Your CPU and motherboard are pretty old, so buying anything more than a 7750 will start to be bottlenecked by the CPU. So a new card will be a decent upgrade, but you'd need to a new build altogether to see a big performance increase.