How much faster do you want to accelerate? Also keep in mind that with shorter diff gears (greater mechanical advantage) you will wheel spin easier so a diff centre upgrade will also be recommended to make use of the increased acceleration. Also keep in mind that with shorter diff gears what you gain in acceleration you loose in top end so car use has to be factored into the equation.
Stock = 3.08 gears
3.45 gear about 215rpm increase @ 100km/h ~ 2000 rpm
3.73 gears about 380 rpm increase @ 100km/h ~ 2200 rpm
3.91 gears about 485 rpm increase @ 100km/h ~ 2300 rpm
4.11 gears about 600 rpm increase @ 100km/h ~ 2400 rpm
For a manual car 3.45 gears work good, for an auto 3.73 are a decent compromise, if all out acceleration is the only goal that 4.11 FTW.