Best to install your GTX1060 first and see how you do.
If you play fast action games, you may be sufficiently pleased.
If you are cpu limited, try to find out if your limitation is because of single thread speeds or the lack of threads.
Look at the processors supported by your motherboard to see what options you have.
Your two thread I3-550 has a passmark rating of 2834 and a single thread rating of 1299.
Not bad for some games.
Look at the passmark ratings for any cpu upgrade options you have. Not likely to do better on the single thread front.
By comparison, You can buy a budget starter cpu like a 4 thread G4600 for $85 with a passmark rating of 5427/2071.