Smoothing groups don't have anything to do with whether it is low or high poly. Take a look at the picture below (I googled "smoothing groups": )
It will help define the model you already have. No need to make it high poly. The model in the above pic are all the same with no additional polys.
Also, the artists behind the most popular graphically superior games on Earth do do everything they can within the limits they have. And smoothing groups is one technique they use.
That is a very nice sword. With a little surface touch ups like what Link102 suggested, it can look super fantastic!
My green thumb grew the tree my Trojan War horse was crafted from. With roses in our pockets we rally round the tombstones. Ashes to ashes, we all fall down.