Also, this site works EXTREMELY well for specs on all Macs and iDevices.
You can look up each Mac, down to serial number and then open a large list of specs.
For your iMac EMC2133
Scroll down a bit and you will find this info:
RAM Type: PC2-5300 DDR2 Min. RAM Speed: 667 MHz
Details: Supports 667 MHz PC2-5300 DDR2 SDRAM (200-pin SO-DIMM).
Standard RAM: 1 GB Maximum RAM: 6 GB*
Details: 1 GB of RAM is installed as a single SO-DIMM module.
*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.
In the US (and many other countries), site sponsor Other World Computing sells memory -- as well as other upgrades -- for this iMac.
In Canada, site sponsor The Mac Market sells memory and other upgrades for this iMac.
In Australia, site sponsor RamCity sells memory and other upgrades for this iMac.
In Southeast Asia, site sponsor SimplyMac.sg sells memory and other upgrades for this iMac.