Just a quick post in case anyone Google's this question. Yes, my MacBook 2.13Ghz with DDR2 6400 800Mhz RAM is quite happily running 6GB of installed RAM (1 x 2GB and 1 x 4GB modules). Not only this, I've tested and made sure that the full amount of RAM is being addressed and used (quite happily opened Adobe Photoshop, Illustrator and Dreamweaver CS4 with several other Apps and at least initially had some memory left over but not much!). If you're interested in doing this upgrade please contact me!