Can it uses DDR2 PC2 6400 memory?

Hi guys,

I'm browsing RAM to upgrade my old mac but rather confuse with either selecting 6400/5300

Mactracker says my processor is Core 2Duo 2ghz Penryn and PC2-5300 (I popped the memory myself to see if it's true)

but IFixit store says that pc2-6400 compatible with : 2 GHz (Early 2009) Core 2 Duo MacBooks. I looked up several sites and it says that as long as it penryn then it'll work.

Now, having higher RAM clock seems nice but will 6400 work ? Can I use 4+2 = 6GB like what many people suggest?

My only fear is that why the original only gave me 5300

Thanks