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The limitation is with your iMac, not Mac OS X. The chipset in your iMac will only support up to 2GB of RAM and that cannot be changed at all. Its just a limitation of the computer.

Mac OS X version 10.4 and 10.5 will support many gigabytes of RAM. Since 10.5 is a 64 bit OS, it can truly support hundreds of gigabytes of RAM.
 
Thanks guys for the information.... I will probably go get 2x2 then.... 3.3 is better than 3.0 :)
 
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