I noticed temp 51C/125F is this normal
Actually it is a bit high but not earth shattering. As long as the system runs stable it shouldn't pose any problem. However, is that under load or an idle temp? If it is an idle temp you might want to take a look at either improving your airflow in the case or getting a better HSF. That volcano might have been "OK" for it's time with the 700, but an upgrade to a newer/improved HSF might be a wise choice now. It doesn't have to be an expensive upgrade either. You can get an excellent HS for under $25 and there are some pretty nice ones for a lot less.
Just for giggles though, you might want to closely examine your installation of the HS again. If it is slightly "off", it would account for the rise in temps. Is the cut-away on the bottom of the HS matched up with the same area of the socket and is it sitting flat on the CPU? How much thermal paste did you use? Too much can insulate the CPU and cause a higher temp. Just something to look at before deciding your next move.