I have been an amateur photographer since I have been a kid but recently have entered the serious amateur category.  I always shot film but bought the Digital Rebel XT almost when they first came out.  My hobby has increased and time to upgrade.  I was wondering for any input on the next step should I go with XTI or Xsi (which offers more than what I currently have and a lot cheaper then when I bought my first digital SLR) or will I even notice more gain in the 40D.  I know I would rather spend money on good lenses and wanted to know if there is significant difference between these three. 

I have been shooting a lot of portraits of children and nature.  Thank you in advance for any feedback and thoughts.