If you are running an older Intel based iMac and would like to increase the speed, you can upgrade the memory from 1GB up to a maximum of 4GB,s. The video below is a very quick guide of this procedure. The 2007 iMac model uses PC2-5300, DDR2 667 MHz specification memory in the SO-DIMM format.
If you own a newer iMac model the procedure is smilier but a different specification memory is required.