From the Boeing 737NG FCTM dated June 30th 2010, pages 1.11 & 1.12
...the recommended method for approach speed correction is to add one half of the reported steady headwind component plus the full gust increment above the steady wind to the reference speed. The minimum command speed setting is VREF + 5 knots.....
When making adjustments for winds, the maximum approach speed should not exceed VREF + 20 knots or landing flap placard speed minus 5 knots, whichever is lower.
And
Note: Do not apply wind additives for tailwinds. Set command speed at VREF+ 5 knots (autothrottle engaged or disconnected).