Hey, I'd like to see that 36' TV. :)
How big was the image you're projecting from the 75U? I run with an 80" diagonal 4:3.
Remember, even though the Xbox can do hi-def, most games (all so far I think) only do 480p. Most games (Halo) don't do fullscreen antialiasing either. So, yes, you see jaggies.
However, if your Xbox thinks your 'TV' is Widescreen, it will output an anamorphic signal for some games (DOA3, Rallisport). This can really help reduce the jaggies.
Sorry, don't have Spiderman, so don't know about any color issues there. None of the games I have played have colors that are 'too dark.'
Which display mode are you using? Natural, Standard or Dynamic?