Yes I was aware it's the glass that's most important, but I assumed that to be in it professionally you needed an fx sensor. I do love my D300 but i find with a lot of my 'informal portraits' I am just not getting pin sharp on the eyes, I only have a nifty fifty (D) version and bog standard 18-105 dx vr so well aware I will need different lenses. By informal portraits i mean taking them in their work place or where they enjoy their hobby or like to relax, really I just mean out of the studio
Getting sharp images has very little to do with the body. It is lenses and technique. Do check the AF accuracy of your lense.
. Just mount the camera on a tripod.
. Place a scale at an incline - the bottom towards you and the top away from you.
. Level the camera so that it is horizontal.
. Change the level of the tripod so that the camera points to the centre of the scale (approximately)
. Choose the larges aperture - F1.8 if it is 50mm F1.8, F1.4 if it is a 50mm F1.4.
. Choose the centre AF point. Note the graduation mark.
. Shoot.
. Now on your computer check whether the graduation you focused at is the sharpest. If AF is off the sharpest focus will be either at the top - back focus or at the bottom - front focus.
In case the focus is not spot on, use the menus to fine tune the lense.
As far as getting professional grade images, again FX has very little to do with it. Many professionals are using DX bodies.