Want a better hq camera, sell or keep my xsi for learning?

I started with an XSi and was quite happy with it.  The image quality will be slightly better with the T3i/T4i... but the difference will be imperceptible to the naked eye.

The lack of flip out screen really only hampered me for a few shoots (to put that in perspective, my shutter count is over 28,000).

As for video, I never cared about it -- at least, not in the sense that I want professional looking video.  I just use my iPhone for brief clips of my sons doing cute things.  If video is important to you then obviously the XSi won't cut it.

Use the XSi, learn what you like about it and what you need in a new camera, and then when you have the money buy a camera that better meets your needs. The best camera, is the one you have with you.