i thought they test the Corsair XMS sticks in the Asus A78NX b/f selling? They are supposed to be optimal for Nforce2.
As to your question, CAS 2 or 2.5 only has to do with the speed you run your memory at. 2 is faster than 2.5 but is more expensive.
I use Corsair XMS and like it alot - very stable