If you guys decide to do a donation to a Children's Hospital- something like a PS3, Wii, or Xbox would probably be more appropriate (I see CP mentioned that). Take donations to buy the hardware then have see if a member/artisan would be willing to give it a special WCG/TPU paint job and you're set.
The full built 'turn-key' rig is still a great idea, although just browsing the FreeDC stats lately doesn't really turn up any possible candidates to take the rig. Maybe a Crunching Contest should be run like before?
I think a good thing to look at before we get the rig idea rolling is to see what happened to the last one. Is the member still here? Still crunching? Did the rig even crunch for X amount of time? Etc. I would be very disappointed to get a full rig together only to have the winner disappear.