Can i replace it with my old.....

[mutiny]

Harddrive, I have a G4 and a macbook with a hd i can not reformat, can i swap the 2 and have osx still run on the macbook pro even know its intel?

 

[Zel]

Nope, the operating system installed on the one is not compatible with the hardware on the other.

Your best bet is to buy an empty external harddrive chassis (USB or firewire) and put your old one in there to use as a data drive.