Ah, I understand. You just want to make sure no data is left on the computer before you sell it. All your data is stored on the hard drive - nothing is permanently stored elsewhere. You don't need to physically remove the hard drive to do this; you can simply completely erase everything on the disk.
However, if I read into your post correctly, you are buying a new Mac, and want to sell your old computer to recoup some of the cost, right? This would mean that you would want to copy all of your important data to the new computer first. Do you have your new computer yet? If you do, you can transfer the data really quite easily. Just turn your old iMac off, and plug it into the new computer through Firewire. Then, turn on the iMac, holding down the 't' key. This makes your iMac act as an external hard drive to your new computer, and will show up as such. Then, copy everything to your heart's delight.
Then to erase everything off of your old iMac, turn it off and disconnect it, and then put in an OS X install CD, and boot it up holding down the 'c' key this time. Once it comes up, go to the 'File' menu and select 'Disk Utility'. Now, you can choose to reformat your hard drive. Simply reformatting is not enough, though, as it just removes the database that tells the computer where things on the disk are. You must overwrite all the data. To do this, once disk utility opens, click on your hard drive. You'll want to be sure to click on the one that lists the size and manufacturer of the drive, instead of the name of the drive (eg "5.6 GB Maxtor <part number>"). Then click on the "Erase" tab, and click options. Select "Zero all data" and "8 Way random write format" - this will not only write over all of your data, but it will randomly write over it eight times, just to make sure that no electro-magnetic traces are left. Once this is done, you can be sure nobody will ever be able to read your data off your disk (and this includes you, so be sure you have off of it what you need).
Hope this helps. If you don't need to backup your data, you can simply skip that part, and the erasing part will still work fine.
If you have any questions about this process or anything that comes up along the way, just ask!