You should take temps doing something else because prime just burn everything.
If your main is gaming you can see that ANY game doesn't need more than %25 load per core.
I use PRIME to test new OC's but not to worry about temps. Of course that if i get 100 loading prime i would worry lol.
I don't think 500w is enough for that rig anyways... but meaby i am wrong
if you wanna be more sure, try lowring the multiplier and mantain your fsb and make it stable and check temps, after that i would increase multiplier and vcore if needed to be more sure.
leave memories on stock for a start