That sounds really low - but that's good. I have an MSI 770 twin frozr and my GPU stays around 70c all the time playing same game at max. Apparently this is normal, and only start worrying when its getting up to 90c.
51c is relatively cool when at full load. Looks like you got a good card. Be happy.
51c is very cool compared to super overclockers who let their cards get up to 85c