You really don't need to worry about a case being able to "handle" the processor. That's a function of the mobo and PSU (giving enough power from the PSU and the mobo being able to use the chip right out of the box). Just get a quality case, power supply, and mobo and you'll be fine. Make sure you get at least a 300-350W PSU (closer to, or over, 400W being better) and you'll be good.
Probably, the reason you're seeing the 3GHz processor spec listed is that's what was the fastest when the case was built. I've not seen a power supply list much in speeds of processors, usually just the family, unless it says "and up" after the speed.
What cases are you looking at???