I doubt Apple is interested in anything as old as a G5. I believe there is somebody on the forums who refurbishes old Macs and donates them to schools - maybe a Discussions search might turn up the name. For many anything older than two years is considered on its way to obsolete. I recall hearing about schools that basically aren't interested in anything that's older than two years, which makes my jaw drop since I have
never owned a computer as young as 2 years.
As for selling, check value on places such as e-bay and Craigslist (CL is good for getting local prices). Also consider what somebody will have to pay to get their hands on a working computer. If I have to pay $40 for shipping, well, I might just look for a computer that is abut $40 more expensive locally since it's all the same in the end (and locally I can at least see it before buying).
If you're going to sell it with an upgraded OS such as Leopard make sure the buyer gets those discs. You're legally required to include them or to erase the upgraded OS. However, even those disc have value. If you're providing a set of retail OS discs, well you could probably get near new value for both Tiger and Leopard retails discs.
No hard drive. Factor in what it'll cost somebody to buy one. For me it is always bottom line cost to
me. If I have to buy a drive, pay shipping, etc. then suddenly your computer may cost more for me to have in-hand and working than a more expensive one that comes ready to work (and I don't have to mess with it).
For what it's worth: Mac2Sell offers online estimates for used Apple Macs
http://www.mac2sell.net
Message was edited by: Limnos