I'll add some additonal advice and help.
If you want to speed up your current iMac, your model can take a total of 8 GBs of RAM memory.
Here are the specs. for the RAM modules you need.
| 8.0 GB |
| Memory Slots |
2 - 204-pin PC3-8500 (1066 MHz) DDR3 SO-DIMM |
You can purchase correct and reliable Mac RAM from online Mac RAM sources Crucial Memory or OWC (macsales.com).
Here is what you need from OWC.
http://eshop.macsales.com/item/Other%20World%20Computing/8566DDR3S8GP/
and how to remove and install new RAM here
http://eshop.macsales.com/installvideos/imac_mid_2007/
Good Luck!