It's common practice to ask for full system specs before suggesting PSU, so that we can suggest you a proper wattage PSU. But since you need a new PSU for a GPU that requires 1x 6-pin PCI-E power connection, that alone says quite a bit.
Any GPU that uses 1x 6-pin PCI-E power connector is under 150W. It can be even as low as 75W GPU. Normal system (not including servers/workstations) consume about 200W, making the total around 350W. 550W PSU is more than enough for such a system.