Can my early 2008 MacBook run Mavericks? I have an early 2008 MacBook with a 2.4 GHz Intel Core 2 Duo and 4 GB of 667 MHz DDR2 RAM running OS X 10.5.8. Can I upgrade to OS X Mavericks? I realize that my early 2008 MacBook isn't on the list, but I have additional memory. Thank you!
No you can't, if your device is not on the list. The minimum RAM required for Maverics is 2GB. Increased RAM will not help you.

By the way here are the supported devices

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