I don't have any firsthand experience with the XT, but from what I understand the autofocus on the XTi is significantly better than it is on the XT, particularly in lower light conditions.
That said, the XTi has the same focusing system as the 30D (which is, I believe the same as on the 20D, actually), but the 30D is much better built. If you're not terribly concerned about how the camera feels in your hand then the XTi should do rather nicely for most things but I found the XTi was far too small for my hands to be comfortable.
You really should go to the local store and handle the XTi and compare it, if possible, with the 30D to see which body size suits you better. Then work from there. The 20D is the same size and build as the 30D so if you like the 30D then you'll probably like the 20D as well -- the only things the 30D really has on the 20D are spot metering, an RGB histogram, and a somewhat larger screen.
While you're likely to keep your glass quite a bit longer than any camera body, you may find that the comfort, build quality, etc. make a big difference in the amount of pleasure you get from taking pictures. I know that's how it works for me, enough so that I went with a 40D because for me the 30D was just ever so slightly too small in my hands.
EDIT: there's not much of a difference between 8 megapixels and 10, really. In linear terms, 10 megapixels is about 12% bigger than 8. And the smaller pixel size of the 10 megapixel sensor places heavier demands on the accuracy of the autofocus system and the sharpness of the lens. Bottom line: I wouldn't bother factoring in the resolution when evaluating these cameras.