First, you should know that X79 is the first (and only) consumer platform with a quad channel memory controller. That means for maximum memory performance you must use at least 4 DIMMs, populating channels A, B, C, and D.
The Sabertooth X79 originally supported up to only 1866 MHz, but I see the QVL lists kits up to 2400.
http://support.asus.com/QVL.aspx?SLanguage=en&p=1&m=SABERTOOTH X79&hashedid=RPOIwd0pKLoXnbFV
If you buy a kit on this list, it has already been tested to work with your board, at least at stock CPU speeds. If you overclock your CPU (and you should with that unlocked processor), some of the higher speed RAM may not boot. I believe you'll be safe buying a 2133 kit, even one that's not on the QVL. I'm running Kingston HyperX Beast 2400 in my Asus P8Z77-V & i5-3570K system with great results, and it's not on my QVL. If you do go for the 2400, you should pick one of the 4x4GB kits from your QVL for assured compatibility. If all those are hard to find (or too expensive), roll the dice and get the kit you want, but make sure it's a name brand like Corsair, Adata, G.Skill, Patriot, Kingston HyperX, Crucial, etc., as they have reputations to preserve and do lots of testing on all platforms.
Also, check out this recent thread about whether or not fast RAM is worth the money -
http://www.techpowerup.com/forums/showthread.php?t=191311