Can my iMac support faster memory?

I have a silver (bought in 2008) 24' iMac 2.8Ghz Intel Core 2 (box says extreme, system info says duo), OS X 10.5.6
RAM is 2GB DDR2 667 (which Im assuming is the PC2-5300)

I want to upgrade to 4GB of RAM. My question is, can I upgrade to the faster DDR3 memory, specifically the PC3-8300 1033? Or, do I have to stick with what the factory has put in?

I did some scans that Crucial offers, and have talked to a couple of dealers and they say I have to stick with the DDR2 memory. One of the dealers told me that the DDR3 PC3-8300 would be the best way to go. So, I'm confused now.

I appreciate any help, advice, success, horror stories.

Cory

Your iMac takes PC2-6400 modules and here is a link to their specifications:-

4.0GB Kit (2.0GB+2.0GB) PC2-6400 DDR2 800... (6400DDR2S4MP) at OWC

Upgrading the speed of the memory will achieve nothing as the iMac will still operate at the PC2-6400 speed. In my FW800 I have PC3200 modules, however Apple System Profiler sees them as PC2700, and that is the speed they work at.