It would help to know more about your goal. Are you trying to replace the Sigma? Add a second lens? Add a lens that is truly useful in low light situations?

The 70-200 f/4 will certainly "work" on the XTI, and it's a fine lens, but it's probably not the best combination. The lack of IS on the XTI will require pretty good light to get adequate sharpness. And if you expect it to be your only lens, 70mm on a crop body is too long.

Please share more about your interests, goals, etc.

Brian