Thermaltake makes more than a few 550w power supplies. I would need the actual model number so I could check the quality of the unit you are thinking about buying.
do NOT buy a thermaltake smart PSU if that is what you want to buy. It is tier 4 and unreliable. Try to find a PSU on pcpartpicker.com that is within your budget and is at least tier 3 but preferably tier 1 or 2
but yes 550 watts is enough for an r9 380
if you tell me your budget ill build you a system on pcpartpicker
Thermaltake makes more than a few 550w power supplies. I would need the actual model number so I could check the quality of the unit you are thinking about buying.