Hi newcomer and welcome to the Tom's hardware forum.
1- AMD site recommend 62C or less, but with 70C or less you are safe.
2- If the 54C are on idle (without heavy work), IMO is high. It's in load, is a little high temperature, but is good.
is a guide to overclock the X4 955, you can follow the same steps to OC your X4 965. I think that you can get 3.8GHz-4.0GHz stable. Maybe more if you can get a correct configuration and a good cooler.
4- Well, the cooler depends of the space in your case. But a very good cooler for that processor is the Coolermaster Hyper 212+, I have one with my X4 955 and I can OC until 4.0GHz stable, you can take a look of my signature. This
is my rig and the process to install that cooler.
Finally, you can see here
a recommended list of AMD coolers. But like I say up, all depends of your case.