How do you Wipe Computer Clean???

Hello-

I bought a new 27" iMac and want to give the old one away to the local library, how do I erase all traces of me and all the files??

The iMac runs great & has had numerous upgrades over it's life. I don't want to "degrade" it but am afraid it may not respond to a standard attempt to erase the system.

Here is what I have:

ORIGINAL BOUGHT AS FOLLOWS: (bought 11/30/2002)
iMac 17" widescreen
MODEL M8812
800 MHz Pwr PC G4
256 MB
80 GB HD
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UPGRADES:
*1024 GB SDRAM
*DAYSTAR MAChSpeed G4 iMac flat panel CPU upgrade; 1.35 GHz (7457 CPU w/ 512 L2 Cache installed).
*OS9 removed and HD not partitioned anymore
*OSX 10.5.8 "Leopard", I believe a "PATCH" was used to allow this iMac to run Leopard.
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**If you select "About This Mac" from the Apple Menu the computer lists the processor as follows: 350 MHz Power PC G4. Processor Name: Power PC 60?(1.2)
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I have all original packaging and all CD's that came with it. I don't want to erase everything to the point that OS9 from the original CD's would have to be loaded again (OS9 was a real drag, never used it).

Any suggestions as to how I can erase most things from this system without messing it up?

If it turns out to be a hassle then I'll junk it, I'm sure it's not worth anything, hardly a chance to sell with the problems of reconfiguring it all-

TIA