It depends on what you want out of a body upgrade. If you want certain features, better build quality, handling, ergonomics, or user interface, then you should pick one of those bodies.

If you want better pictures, then do not upgrade your body. Given that you only have the XSi and the kit lens(es), I think you would get a lot more out of buying better glass and a flash for your XSi. With the kit lens, you are severely holding back your XSi, especially in low light (unless you have a nice flash to bounce at parties). The XSi is capable of stunning images.

Glass, technique, and post processing are the most important when taking the shots you have mentioned. In sports, you do get limited earlier by a lower body for predictive AF, but an entry level with great glass can still produce stunning sports photos. And a body like the 7D will still produce stunning portraits, landscapes, and low light party photos.

It depends on what you want out of a body upgrade. If you want certain features, better build quality, handling, ergonomics, or user interface, then you should pick one of those bodies.

If you want better pictures, then do not upgrade your body. I think you would get a lot more out of buying better glass and a lighting setup(for portraits and indoor) for your XSi. With the kit lens, you are severely holding back your XSi. The XSi is capable of stunning images.

Glass, technique, and post processing are the most important when taking the shots you have mentioned. In sports, you do get limited earlier by a lower body for predictive AF, but an entry level with great glass can still produce stunning sports photos. And a body like the 7D will still produce stunning portraits, landscapes, and low light party photos.