Jim Strutton , Jul 30, 2008; 12:45 p.m.
You need to experiment with this effect to get it right. Basically you are talking about depth of field and the plane of focus within that depth shifts basically on two parameters, the distance from the subject and the aperture setting. Back when I was taught photography, just a bit before Nelson lost his eye :-) , we would set up various articles along a table and but focussing on the item in the middle distance we would take a series of test shots at different apertures to learn exactly what was in focus at what fstop.
Just remember you need to get sharp focus on the eyes for a good portrait, so I am guessing you are going to want to shoot in Aperture Priority and experiment from there. With a digital camera you can see the difference in the shots pretty much instantly, so as you are unsure, try bracketing, i.e. take several shots either closing down or opening up the aperture as you go. My best guess is that you will end up in the F4 to F5.6 range, but give it a try. Or as my old teacher the late great John Taylor would say, if you want to know if it is going to be sharp then suck it and see!
Hope this helps