If you're planning to try the public Beta of OS X Yosemite, you'll want to refer to the Macintosh System requirements. Plus, we've included some helpful Yosemite links, all in one place.
The OS X 10.10 hardware requirements are, as cited by Apple Insider:
- Mac mini: Early 2009 or newer.
- iMac: Mid 2007 or newer.
- Mac Pro: Early 2008 or newer.
- MacBook: Later 2008 Aluminum or early 2009 or newer.
- MacBook Air: Late 2008 or newer.
- MacBook Pro: Mid/Late 2007 or newer.
- Xserve: Early 2009.
This is just about the same as the OS X Mountain Lion and Mavericks System Requirements except for some slight changes regarding the MacBook Pro.
Here are some other handy links.
MacBook Air with Yosemite via Apple.