What should I upgrade to?

[moquinj]

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. 

[Bob Atkins]

I'd go to a 40D if you have $950 to spend, or a 50D if you have $1400 to spend.

While there's veru little difference in image quality between the XSi, XTi and the 40D, I think the 40D handles a little better and I is a nicer cameras to use. The rear QCD (quick control dial) is useful and I llike the LCD on the "top deck".