How much RAM can I install?

Hi.

I have and iMac 20-inch early 2008 MB 324E/A

Intel Core Duo ("Penryn") 2.66GHz

I use PC-26400 (800MHz) DDR2 RAM SO-DIMM.

I used to have 2GB. According to some info I got, the maximum amount of RAM I could install was 4GB. So I did.

However, I recently used the Mactracker app, and under the "Maximum RAM" info, it says: 6.0GB (Actual) 4.0GB (Apple)

What does this mean? That I can install 6GB?

If so, how? Because I only have 2 RAM slots. That would mean that one must have a 2GB DIMM, and the other one a 6GB? Or use only one slot with one 6GB DIMM?

I am really interested in knowing this, as I use RAM-intensive applications: Photoshop, Illustrator, Final Cut, etc.

Thank you