I have the 17-55/2.8 for the Canon and absolutely love it. But it is expensive, heavy and large and I suspect if loaded up onto an XSI (which is small) might not make the camera holdable - I have large hands and find the XSI grip is too narrow as it is for my fingers - adding a fat lens such as the 17-55 would not help (this lens is fine on my 40D which is a larger camera).
If you are considering full frame, any of the three lenses you listed are EF-S style (I am assuming the Tamron is as well) and these lenses would not work with a full frame camera body so if you think an FF body is coming your way in the near future you need to keep that in mind as well in your decision making process.
And I am not sure where you are coming from with the manual focus limitation - You can manual focus with any of these cameras (XSI, 7D, 5d's, etc.). The lenses have a switch on them allowing you to choose MF/AF mode. And if you meant to say the XSI cannot do manual mode (meaning you have full control of aperture, ISO, shutter speed, etc.) it can do that as well as all the various auto modes. It just won't be as feature rich or robust as the 7D, %D's you were also naming.
Hope that helped