Please watch this install video for instructions:
RAM is hardware, it's relatively inexpensive and if you can turn a screwdriver you can do the job. I would also recommend purchasing the RAM from OWC (www.macsales.com), they specialize in Macs and have would class service and support.
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If you have an Early 2008 iMac, you can install 6 GB of memory. You can buy it in OWC or Crucial > http://eshop.macsales.com/shop/memory/iMac/Intel_Core_2_Duo_PC2-6400
You can install more memory on your iMac without any problem, so don't worry about damaging the Mac. See > http://support.apple.com/kb/HT1423#2