If you have the service manual or Google-Fu the hard drive is the third easiest component to upgrade after the RAM and wireless card. That is, after you take the LCD off.Oh, I know, I was addressing the fact he said RAM, is the current iMac's hard drive even user serviceable? If it is, I learned something new today.
Note: This is based off of my service manuals for the Core Duo/Core 2 Duo (Napa) based iMacs. I don't know how much this still applies to the Santa Rosa based iMac.