No one can answer your question.
Why wait? Always something will be new and supposedly better. The XTi/400D is a proven performer. How much better must a camera be for you decide it is worth a purchase?
You will never know until you start.
Buy a camera with a kit lens and it may be all you ever need.
Again no one can answer what the DIGIC III will be like for the replacement to the XTi/400D, only speculate or go by what information has been put out by Canon themself.