Memtest test 7 stresses ram and NB, if it errors out try upping the NB voltage and try to play a bit with the NB GTL values... How many passes of memtest are we talking about ? if you encountered this same behaviour on the P35 then it could well be that the CPU has issues mate... though 450FSB really isn't anything special... what happens if you try 445FSB ? ram at 1066Mhz
Disable the sound and the aero glass theme ( both options can be selected in the hyperpi program : top right )
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