Anything above 6mp is a novelty unless you're a paid professional doing prints larger than 8.5x11. If you keep all your prints smaller than 8.5x11 then larger MP will only benefit you if you're a fan of cropping, in fact less MP improves high ISO noise. In the world of dSLRs better glass is a better investment than a body in most cases. Unless you're going from a D40 to a D3. The lens controls the sharpness of the photo, the colors, the bokeh, fast autofocusing to capture moving shots before they're out of frame. You can have a camera with the fastest shutter response ever but if you put a lens in front of it with a slow, clunky AF than you'll probably miss the shot.
To the OP, if you're just getting introduced to the dSLR side of photograph then I'd suggest finding a new or used D40 or Rebel XT. You'll find no real benefit of upgrading from these cameras to the 40x/60 or XTi/XSi when you're just starting out. And when you do finally discover the differences, they'll be so minimal that you'll want something more powerful anyways.
To go along with the thread, I shoot with a Nikon D50 with a Sigma 18-50mm f/2.8 and an SB-400 with diffuser. You can see some of my pics here:
Flickr: arisenfury's Photostream