You didnt list it in the New Build list but I'm assuming you included the 560ti. That said, the 550w PSU is plenty as nVidia recommend 500w for that card and your PSU is a good one. However, there is just a possibility that it has been damaged during the rebuild.
Now tell us a little more about 'machine starts to loop' - do you mean keeps rebooting or something else?
if that doesn't have any effect I'm going to pick up a 700watt PSU which will provide a good alternate PSU
Your system should use around 300W peak so your PSU problem (assuming the PSU is at fault) would be quality, not quantity.
I'm guessing your Antec PSU is 5+ years old, which would be around the time Antec's OEMs were (still) using Fuhjyuu or similar low-quality caps. In that case, your PSU is overdue for a re-capping after which it would be fine for many more years.
After reading thru a bunch of Newegg reviews I'm more inclined to believe it's the motherboard than the power supply
You might want to read thru them ( lots of boot loops involved ) and maybe get in touch with Gigabyte support prior to replacing a good quality psu for one with more wattage that you don't need http://www.newegg.com/Product/Product.aspx?Item=N82E168...