Okay that puts things into prospective, I don't foresee making the jump to FF due to the high costs of the body's 1500 is probably the max I could ever spend on a camera. and I enjoy being able to shoot really wide when I want to which probably rules out all L glass for me.
do you currently have an EF-S 18-55 kit lens with your XSI?
If so, check it out @ 18mm ... and/or check the EXIFs of your pictures (there is free software to do this) to see what focal lengths you are mainly using ... and decide yourself whether 18mm focal length is "really wide" for you.
If the answer is NO - and you want to it even "wider", then look at ultrawideangle lenses, e.g. the EF-S 10-22 or Tokina 11-16 etc.
If 18mm is "wide enough" for you ... then the next question is: do you intend to frequently shoot moving subjects (people/children, animals/pets, vehicles, etc.) often also in rather low light (e.g. everything indoor)?
If yes, you may want to look at the EF-S 17-55 which offers a very useful focal range and a wider aperture (plus Image Stabilizer) on top. It is quite fat, quite heavy and not cheap, but performance-wise worth every cent in my opinion.
If you want somewhat more "reach" on the tele end [again, check with the kitlens @55mm] and if you plan to use the lens almost always in good lighting conditions (=outdoor, fair weather, flash, studio), then look at the EF-S 15-85 IS.