-I'd trust the coretemp readings. uGuru is probably reading the temp from the socket area which is always cooler.

-Your temps are OK but run small FFT again with 4 threads of Prime95 and see what happens. If it hits 70c then you probably can't OC any higher.

-to improve performance, you can set the CPU multiplier to 7 and the FSB to 463MHz and adjust the RAM to a 1:1 ratio That'll give you the same CPU speed you have now but much higher bandwidth on the FSB and RAM. Run Prime95 on blend for a few hours to see if it's stable.

-if it pases the previous step you could try a higher RAM divider to speed up the RAM. I believe the next step up would be the 4:5 divider which would put the RAM at DDR2-1158 which might be too high. If it is then you could stick with 1:1 and try tightening the timings, 4-4-4-12 for example.