No.
On the web or prints no larger than A4 you'll have no benefits with higher resolution sensor. However if you crop your images, there is a big difference in having a 2 megapixel or a 6 megapixel cropped result regarding how much detail is left in the image.
I don't think you should consider an upgrade for the megapixels alone. The K-7 is for instance a much more solid body, a lot faster autofocusing, better auto white balance, a lot better lcd screen, two control wheels (more direct control without looking for buttons), faster and more responsive menus, live view, much larger buffer (don't have to wait that much), weather sealing etc.
I've barey used the K-x and to me it didn't feel all that different from my K100, but it surely captures more details if you zoom in to 1:1.
The K100D is a very nice little camera capable of excellent image quality. Some still praise it for its vivid colors and noise free images even at largish iso values. I warn you however - if you buy a K-7 you'll never look back regarding how it handles and feels in your hand each time you're out shooting.. (but you might remember how much fun it was back when 6 megapixel images were completely noise free at 1:1 zoom level)