I used to have a Athlon 4000+ dual core and that worked ok with dolphin, just get a decent graphics card. A Geforce 8xxx or 9xxx should do. Don't overclock if you don't know what you are doing and especially not if you are already having problems with overheating.
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VISTA ULTIMATE x64
AMD PHENOM II 940 X4 @3.8GHz
4 GB DDR2
GTX 260 core 216