Pretty sure that the GPU miner for Primecoin won't be any faster. It's finding prime numbers, using basic arithmetic. This is what CPUs excel at.
That's still theory, Litecoin at the beginning also was thought to be exclusive to CPU only.
Well, I really don't see how a GPU would possibly be faster at that type of math, but OK.
An amd 7970 has 2048 cores that can all do arithmetic, a core i7 has 4-6 hyperthreaded cores. The 7970 wins by a long shot when is comes to just doing arithmatic. As far as I understand the weakness of gpu's is that they cannot handle code that has to make if else decisions well.
Bitcoin hashing is just basic arithmatic as well.
Offloading the primality checking to a GPU should be easy though.