Yes, 51C for a thermal margin is very good...although that's probably at idle. You'll want to check your margins when you're using it -- don't be surprised if you see that margin dip down to 10-20C.
EDIT: AMD Overdrive is much more reliable than a lot of the other temperature-monitoring software, as it tells you exactly how much is left (i.e. when your thermal margin hits 0C, it means you're at the safe limit). This is in contrast to some others; for example, I also use CoreTemp to monitor, but a) it measures CPU temp instead of the margin, & b) any temps it reads that are under 40C are actually unreliable, due to how it reads from the sensor.