3 final choices: Rebel XT, Nikon D40, Nikon D50?

For around $600, I can get either 3. XT is an obvious choice but I STILL seem to gravitate towards Nikon, however it has only 6.1 compared to the 8 of the Canon. Will I see a big difference in the two? I want SOLID camera I can grow with and that means lenses I might buy. Don't care about bells and whistles but a solid camera with manual controls to take the pics and experiment with. What's the best long-term camera? The D40, I hear might be less flexible than the D50 but is the D50 obsolete? What about the D40? Solid camera? XT? I'm concerned it's a flimsy peice of plastic with excellent componants. I'd prefer a more solid frame.. Suggestions?