Indeed, Pentax is not known for great low light AF performance. But there are a couple of things to consider:
- You mention the FA35/2. Well, DOF at f/2 is thinner than at f/2.8. Enough so that it is indeed tougher to get both eyes and nose in focus.
- The light in the store was probably better than what you are comparing to in ordinary home settings, so unless you were specifically comparing your camera in the same store, it might not have been a fair comparison that way, either.
- You say you tried the 24-70/2.8 VR. As far as I know, there is no such lens. The only 24-70 that shows up in my searches has no VR, and the discussions that turned up consisted of people doubting there ever would be one. Something to consider, after getting spoiled by having SR on every lens for Pentax: you'll have to live without it for most lenses with Nikon.
But there is also no getting around the fact that the D700 has lots more focus points and a more modern AF system - meaning it is more likely to choose the "correct" subject to focus on. And that it has a built-in AF assist lamp, meaning the AF will be faster and more accurate in low light, too.
Quote: How can I make my K100D focus faster and better in low light?
You could use a lens that is known to focus faster than the FA35, such as the DA40. You can use center-point focusing and point the camera directly at the subject rather than make the camera guess, which takes time, and also might lead to it guessing wrong (eg, the plant instead of the person). You can always then recompose as desired, and possibly touch up the focus with quick shift (the latter also requires a DA lens).
You could also work on your MF skills. Most of my low-light shooting is done with old MF lenses. With candids, I focus before I need to take the shot, then wait for the right moment to snap. You could also consider the Katz Eye focus screen or the O-ME53 magnifying eyecup to aid in manually focusing.
And you can hope that in six months, if you are considering getting a new camera, Pentax may have something with a much improved AF system. Personally, with my DS, I lost *far* more shots to shake than I did to poor focus. You couldn't pay me to go back to a camera body with no built-in stabilization.