To run 10.9 you need a 64-bit EFI in your Mac.
Here is the official list:
• 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
– 15-inch, Mid/Late 2007 or later
— 17-inch, Late 2007 or later
• MacBook Air — Late 2008 or later
• Mac Mini — Early 2009 or later
• Mac Pro — Early 2008 or later
1st-gen plastic Macbooks are not compatible
(shame, they are great machines)