If you were running prime when you took the readings with sensorview then the system was not at rest and the 52C when prime is running is in the normal range because prime puts a heavy load on it.
I tend to think you were running prime when you took the readings because of the cpu and memory usage.
Prime will almost always fail when the cpu temp hits 60C.
Look closer on the memtest site they have a version that can be run from a bootable cd but you have to use a program such as nero to create a bootable cd and copy the iso image to it.
A better way would be to get UBCD for free it contains memtest and a lot of other goodies, look under the FAQ's for directions
http://www.ultimatebootcd.com/