Just booted the machine and fan at idle certainly sounds healthy (I can hear the hard drive seeking over the noise of the fan with my ear right up to the back of the machine).
From Apple's site
Before you install Yosemite, make sure you have one of these Macs:
iMac (Mid-2007 or newer)
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
MacBook Pro (Mid/Late 2007 or newer)
MacBook Air (Late 2008 or newer)
Mac mini (Early 2009 or newer)
Mac Pro (Early 2008 or newer)
Xserve (Early 2009)
Your Mac also needs:
OS X Mavericks, Mountain Lion, Lion, or Snow Leopard v10.6.8, already installed
2 GB or more of memory
8 GB or more of available disk space