That card is really old which means its thermal paste to. I also had a gtx 550ti and as I desembled it the thermal paste was really dry and hard. It took me a hour to remove that. Just buy some thermal paste (usually costs like 5 dollars or cheaper), screw your card open, wipe the thermal paste of the gpu (If the old paste is dry than try using some alcohol), put a pee size of new thermal paste on the gpu and than screw it back up.
Here is a guide: https://www.youtube.com/watch?v=qDLQ7FjPMf8&t=245s