The OS does not determine the maximum amount of RAM your system can take, its usually limited by the processor. 32-bit processors like the PowerPC G4 and the original Intel Core Duo can support a maximum of 4 GB of memory, while 64-bit processors like the G5 and Intel Core 2 Duo can support quite a bit more.
Correct. The 2.16 ghz intel core 2 duo imac's only support up to 3 gb of ram. I'm not sure if upgrading the OS will allow there to be a ram upgrade higher than 3 GB though.
You can put 4GB but only 3.3 will be addressable. It's a limitation of the CPU and chipset (intel Merom, I think). The current platform (last plastic Macbook) runs the intel Santa Rosa chipset which boost addressable memory to 4GB, GMA X3100 graphics and 800Mhz FS Bus rather than 667Mhz.
I think the Santa Rosa chipset started at 2.2, but there may have been a limited release in 2.0Ghz to begin the series.