I agree that the 2006 machine is simply getting too old to upgrade. Donate it to a charity or give it someone who home schools and has small kids who can use it for a couple of years.
I would not invest more in a machine too old for running Mavericks (or future OSs), even if I did not plan to run Mavericks anytime soon.
OS X Mavericks: System Requirements
Learn about the system requirements for OS X Mavericks.
To install Mavericks, you need one of these Macs:
iMac (Mid-2007 or later)
MacBook (13-inch Aluminum, Late 2008), (13-inch, Early 2009 or later)
MacBook Pro (13-inch, Mid-2009 or later),
MacBook Pro (15-inch or 17-inch, Mid/Late 2007 or later)
MacBook Air (Late 2008 or later)
Mac mini (Early 2009 or later)
Mac Pro (Early 2008 or later)
Xserve (Early 2009)
Your Mac also needs:
OS X Mountain Lion, Lion, or Snow Leopard v10.6.8 already installed
2 GB or more of memory
8 GB or more of available space