I just thought I’d share this since I had trouble confirming it myself. There are rumours online that the mid-2007 Macbook Pro (Model A1226) can support up to 6GB of RAM. I have been using 4GB and the possibility of an extra 2GB could not be dismissed out of hand.
I ordered the cheapest possible 4GB RAM module from Ebay. Thankfully it was compatible. Apple computers can be a bit finicky with RAM.