Eric Teel
David --
We handle a fair number of donations at my work.(Jefferson Public Radio - www.ijpr.org - gratuitous plug plug plug)
The IRS basically allows the person donating the item(s) to determine the current value. So, it's left somewhat to the morality of the individual to set a fair value. But... perhaps more importantly, that dollar figure is one that you're going to want to be able to defend if ever asked by an IRS representative.
The advice I'd give is this: Search Ebay and Craigslist and anywhere else you can for similar computer systems for sale. Find the one that has the highest price tag on it. Print the webpage and stick it with your tax paperwork and then claim that $ amount as the value of your donation. Your printout will at least be a way of supporting how you came to that value number if you're ever asked in the future.
Eric Teel
P.S. No, we don't want your computer. =)