Under the "Rec. Mode" menu, there is an "AF Mode" option. If this is set to AF.S, the camera will refuse to take a picture if it can't lock focus. If it's set to AF.C, it will try to focus on the half press, try to maintain focus as long as you're half-pressing, and will take a photo no matter how off it is on the full press.
I can throw in my comparison for the k100d to a k5iis - it's a night and day difference. AF went from something I mostly gave up on, to something I use regularly. It's not the fastest thing out there, but it works in pretty low light and when it locks focus it generally does a great job of it.
I think the k5ii and k5iis had some AF improvements over the k5 and the k3 is even better, how much $$ this is worth is a tough question. Hopefully you'll get a chance to do some hands on comparisons for yourself, that's really the best way to decide
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