xfx 550w will do fine.in fact it's a bit overkill for single 550 ti.
For a system using a single GeForce GTX 550 Ti graphics card NVIDIA specifies a minimum of a 400 Watt or greater power supply that has a combined +12 Volt continuous current rating of 24 Amps or greater and that has at least one 6-pin PCI Express supplementary power connector.
Total Power Supply Wattage is NOT a crucial factor in power supply selection!!! Total Continuous Amperage Available on the +12V Rail(s) is the most important.
I think i will get the raidmax hybrid 2 RX-630SS 620w modular LED power supply. Is this a decent power supply?
No! You want to stick with a good quality PSU. See my signature for a link to recommended PSUs. In general, Seasonic, Corsair, Antec, Silverstone, and Enermax are great PSUs. There are exceptions to every rule...