Originally Posted by Soulwar

I'm assuming you have the 2.16 GHz 24" iMac?
Some info about the ram.

"The iMac "Core 2 Duo" 2.0 17-Inch, 2.16 20-Inch, and 2.16 24-Inch -- the "iMac (Late 2006)" models -- shipped with "at least 512 MB DDR2 SDRAM installed into the top slot". There is not a performance penalty for populating a single slot or populating both slots with different capacity modules.

However, rather unusually, Apple notes that:

Although these iMacs will accept up to a 2 GB SO-DIMM in each of the two memory slots, the iMac will only support 3 GB total memory.
If you want to maximize the amount of SDRAM in your computer, install a 2 GB SO-DIMM in one slot and a 1 GB SO-DIMM in the other. . .
If you do install a 2 GB SO-DIMM in both the bottom and top memory slots of the computer, the About This Mac window and Apple System Profiler will both show that you have 4 GB of SDRAM installed. However, Activity Monitor and other similar applications will reveal that only 3 GB of SDRAM has been addressed for use by the computer."

http://eshop.macsales.com/item/Other.../53IM2DDR3GBK/

OWC also has a "memory trade in rebate".
http://eshop.macsales.com/money-back-rebate