That depends on the stock of the tank. If the tank is full of messy fish (live bearers, goldies, Africans etc) then you should over filter the tank. If it's just low bioload fish then a filter rated for your tank size should be enough. You should also look into what type of flow your stock prefers. I generally like to go a size up from what the tank is.
Also look at the gallons filtered per hour (gph). You'd want to have around 8 times the turn over rate, so for a 160L (40gal) you would want somewhere around 320gph. Again, this depends on the stocking of the tank.