If you put in a CAS3 module and leave the motherboard to auto-detect the memory settings (on most motherboards, option "auto" in the menu), then it will run both the CAS2 and the CAS3 module at CAS3. If you override the default settings and run the memory at CAS2, then you will overclock the CAS3 module.
As far as what you should do, the difference between CAS2 and CAS3 is pretty insignificant. I'd get the CAS3 and pocket the money.