here are my benchmark results and i can tell you that with vsync on my fps may be lower but at least consistent (thus feeling smoother) versus vsync off where I get higher fps but higher fluctuations. I would also suggest you try to check your AA settings maybe its set too high?
1680x1050
MSAA 4x
MSAA quality 2x
Anisotropic Filtering 16x (forced via nvidia CP)
All high (Player detail ultra high)
All ON
vsync on
2009-06-04 00:16:44 - nba2k9
Frames: 2058 - Time: 34412ms - Avg: 59.804 - Min: 57 - Max: 61
vsync off
2009-06-04 00:22:22 - nba2k9
Frames: 1974 - Time: 29248ms - Avg: 67.491 - Min: 52 - Max: 88