cpu maxing out problem

Sometimes cpu will max out for no reason at times

windows 7 32bit
amd athlon 64 x2 6000 3.2ghz dual core
xfx radeon hd 4870 1gb
4gb ram (3 1gb sticks of 800mhz, 1 1gb of 533mhz)
asrock a780gmh/128m board

I have tried taking out the 533mhz and runs only a tad better have even tried putting the 533mhz stick in the first or last ram slot and still having the same issue, I have been told to get a newer graphics card and would like to be sure if that was the problem or not.

I also would not mix an match memory frequencies either.

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