I am thinking to change my motherboard ASUS P5K-VM (intel G33) with the ASUS P5Q-VM (intel G45) that support PCIe 2.0 x16. Do I have to reformat (I have vista 64bit) also do I need to re-activate the windows after this.
You don't need to reformat, but it's usually a good idea when changing motherboards. Before you shut down with the old board, uninstall the chipset drivers. Pop in the new board and install the new drivers. In fact, you should probably do the same with you video card drivers as well. As far as reactivating Windows, I'm not positive, but you shouldn't need to.
Again, best practice and for best performance, you really should go for a clean install.