David Margulius wrote a good piece on how discarded hardware can hurt you or your company.
Last month, Dell provided a "Drop-Off Day" in Austin where you could drop off any old computers, monitors, printers and etc. I have also donated old equipment to Goodwill.
Do make sure that you clean ALL DATA off your hard drives. Simply reformatting the drive will not suffice. I have used Darik's Boot and Nuke (DBAN) disks to clean and wipe my drives. DBAN is a great tool provided at a great price ($0.00). DBAN also provides bootable CDROMs if your newer machine is missing a floppy drive.