Tom congrats on the Xsi. I bought the T1i over the summer and having a blast with it. There is annother forum on the ne, called 'Photography On The net', great information in there.. To me, it is like this forum for photography, it is centered on Canon. Want to feel you know nothing though, wow, these guys have some amazing shots. Other Tom, do yo go on there? They also have weekly theme contests which are fun to watch.
I had always shot Minolta in my SLR dys, and was leaning towards Nikon for a DSLR, no particular reason other than brand impression. Then Canoon came to Yellowstone with the Photography in the Park program and I had that chance to use different cameras and lens, alongside the pros. I was very impressed with the T1i. Also the 50D. They didn't have the XSi available, but I would have gone for the higher pixels anyway.
I have the same two lens you have (Tom), and very happy with them. In fact on the POTN there is a long running thread for just the 55-250, so you can see the capabilities.
You can submit photos, just need thick skin. Good learning points though, including like what Marsha mentioned, one saying you got it right, another teilling you crop. Take it all in and learn.