As background, this controls the number of frames rendered before they're needed for the display. From what I read, a lower number (never use zero, though) reduces latency (i.e. the time from when you make a control movement until it shows up on screen) for faster games. A higher number is supposed to give you a smoother but lower frame rate. With DirectX 9, three is the maximum, so setting to eight won't help anyway. Also the effects of this setting can change depending on whether or not you're CPU bound. With OFF, that is fairly probably, unless you have an older videocard.
So, results can vary, or so I'm told. I tried different settings, and 2 worked best for me. At least that's what I've convinced myself of, anyway.
You'll have to try it yourself to see what works best on your machine. Default is three (i.e. triple buffering). If I have been correctly informed, higher than that won't help with a DirectX 9 game. Setting zero is apparently not a good idea, and can lead to crashes and other oddities. So, that leaves one and two as settings to play with. For me, two helped, but for any other machine . . . who knows?