Yes you can change the RAM with faster RAM. But each module will go as fast as the BUS allows it !
So the speed will not be affected, although you will have 2 modules working in DUAL CHANNEL, which is working a little faster and more efficient than 1 module !
(NOT all Motherboards support this feature. To be sure: check with your MOBO-details if faster RAM is supported. After all it is APPLE !!!!!)
I have also a MacBook from that age and it works with 2 modules of 2GB but maybe the same speed. It runs not too fast but it still works, I'm planning to put an SSD in it, a little gain in speed again.