This memory http://www.amazon.com/Kingston-Modules-1066MHz-KTA-MB1066K2-8G/dp/B001PS9UKW should work with Macbook 5.1 A lot of people confirms that in comments. They also mention Apple updated firmware.
I will find local vendor and order them. Hope they will work.
Do you have a 2.0 or 2.4 processor in your Macbook 5.1? I know we have confirmation from someone in this thread that it worked fine on the 2.4 but I don't have a confirmation that it worked on the 2.0. I am nervous to purchase the 8GB upgrade because if it doesn't work, I don't want to go through the hassle of returning it. I want to purchase reputable RAM so that there are no issues.