But for studio photography, I believe you do not need fast lens. ...
It depends a lot on the type of studio work you intend to do.
In general, you don't need a fast lens from an exposure standpoint as the studio lighting will generally be bright enough to allow you to work at middle apertures. On the other hand, if you are manually focusing you'll find a fast lens a great boon. Also, you may want a fast lens to achieve shallow depth of field.
denawayne:
The type of studio work you intend to do and the size of the space you can dedicate to the studio are the biggest factors to consider in choosing a lens. For portrait work and fashion, if you have enough space, you generally want a lens with a focal length about 2-3x "normal", where "normal" is defined as a focal length equal to the diagonal measure of the sensor. For Nikon DX, "normal" is approximately 28mm, so you should consider lenses in the 60-100mm range. For smaller studios combined with looser framed portraits or full length fashion, a 50mm lens can be a good choice. If still life and product photography are the interest, you should consider lenses like the Micro-Nikkor 60mm f/2.8, the newest is an AFS lenes.
I would suggest that you start by using the lenses you have and, after a little experience, reviewing the shots that work well with an eye to what focal length was used (easily seen by viewing the EXIF data in the image file). After that review, you will be in a better position to judge what new lens to acquire.