When I have questions about the specification or capabilities of a Mac the first place I usually look is www.everymac.com. (FWIW, the site is sponsored by OWC which was mentioned in another answer to this question.)
I believe your iMac should be one of the ones listed below. All of them have this note:
Apple officially supports 4 GB of RAM, but third-parties have been able to upgrade the system to 6 GB of RAM using one 2 GB and one 4 GB memory module.
Apple iMac "Core 2 Duo" 2.0 20-Inch (Al) Specs
Apple iMac "Core 2 Duo" 2.4 20-Inch (Al) Specs
Apple iMac "Core 2 Duo" 2.4 24-Inch (Al) Specs
BTW, there really is no difference between Snow Leopard & Leopard in terms of how much physical memory can be (usefully) installed into a Mac. What matters is the hardware, the memory controller used, not the operating system.
Yes, in theory, Snow Leopard on some Macs allows the kernel to use more than 4GiB of RAM. In practice, most Macs will still use the 32-bit kernel and so have the same kernel address limits as Leopard. (IMO, Snow Leopard is still "better" though for other reasons.)