Originally posted by FrumPilot The k-x doesn't always nail the the focus.
If the only reason you have to upgrade is for better AF accuracy then I think it might be worthwhile to refine your focusing technique first. You can get much more accurate and consistent focus when you know, and control, how your camera (any brand) behaves in different situations. The lens, the light and the subject are also key factors, and your experience and technique is more important than have a faster AF system.
If you are shooting portraits, then I'm assuming you may be shooting in low light and as you are not using a particularly fast lens then it is not surprising that AF performance is not great.
For portraits, I would suggest selecting a single AF point (SEL) and focus specifically on one eye (usually the closest one). AUTO mode will often focus on whatever is closest (nose) or the highest contrast (outline of head). For pinpoint focus control I always use a single AF point.
Also, I find that if the camera has not obtained an accurate focus first time then quickly focus on something else (maybe even your hand) and then refocus back on the eye.
I would try a few things and see what works best for you before deciding you need a better AF system. I hope that helps.