just a breif reply to your point about converting to Poly's from Nurbs if you have a look at your Nurb form as to where the vertices are you will see them floating off the edge. A nurb is using these points to plot a curve so they influence the curve rather than sit on it where as a polygon is using these vertices as absolute points of conection between straight edges. So if you have pulled your original Nurb form hard to acheive the shape you wanted you will often have a few points poking out of your new polygon its easy to correct tho and the speed gain from Nurbs is well worth the oddity.
this should address your other question also about geometry and impact on render ... isoparms in nurbs equal edges in polygons equals more geometry ... so yes it will help to round off / smooth off your form but will add complexity to both further modeling and texturing and rendering ... it is a balancing act to get only as much as you need and where you need it you really have to plan it out ... there is lots of help out there tho so don,t fret