The 1.2Ghz Athlon produces a lot of heat. A headhog with a ducted 92mm case fan might keep it cool enough to hit 1.3-1.4Ghz, but for the best results, you either sink $650 into a case and get a VapoChile case with active refrigerant cooling to about -10'C or you set up a water cooling system with peltiers.
With something like a FOP32-1 youmight get to 1.3Ghz, but you will be running a bit on the warm side.
You also need good case cooling. A blow hole in the side and top along with the fan up front and in back will help, but the best way is to duct the heatsink to a fan mounted in the back or side of the case to draw the heat directly out. Then add a slot cooler to remove the heat from the graphics card. Bay coolers might help too. If you use water cooling you can also cool the graphics card that way (dont try peltiers though).
The COMPUTER is your FRIEND!
Happiness is manditory.
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