The general rule of thumb for portraiture is to use approx a 100mm lens. This usually give the most pleasing effect.
I am not familiar with your camera. If it is a digital you may have to factor in a conversion of approximately 1.6x the focal length of any lens you use. That means that if you use a 100mm lens it would act like a 160mm lens.
If that is the case your current lens should be fine.