I'll try to answer some of them, I'm a nVidia fan myself so not too familiar with ati but I do know something.
Yes, the x1800 series is quite close to the 7800 series
Same thing with the x1900 and 7900
And the xt and xtx versions are usually quite similar, atleast with the x1900 series I don't think there is alot of difference, the xtx has a little bit faster clock speeds, and maybe some other small differences.
Yeah, thats about it. And well, as I said I'm a nVidia fan so I'd suggest u to stick with nVidia