How much RAM can a 2007 iMac hold? The box says I can add up to 3 GB (for a total of 4GB), but is this restriction hardware based, or because of the operating system?

If I upgrade to OS X 10.6 or higher, can I increase the RAM to 8 GB like the newer models?
According to MacTracker, it looks like Apple officially supports a 4GB configuration, but it can hold up to 6GB. (This is for model "iMac8,1").