Well all rumors are saying that there is a Canon press announcement on the 19th
Wait and see what is coming, if the 60D (or what ever replacement) shows up, will mean that the prices of 40 and 50D will drop lower not long after.
In my opinion:
I had a XSI and went to a 40D and loved it
Love the ergonomics, speed etc
Image quality wise I didnt get much difference, the 12 mp of the XSI was actually an advantage when cropping IMHO, I dont think it should be very diffferent from a XSI to a 50D either, more cropping power maybe?
The noise is not an issue until after 800 ISO, and that counts for the XSI, the 40D as well.
I used a 50D once and really liked it for what it counts, the screen was very nice
If i could switch my 40D for a 50D I would do it in a sec, , with the micro adjust the 15mp of cropping resolution and the nice screen, but I just dont need it right now ,it all comes down to money as i would have to put around 400$ on top my 40D.
But I think you will love your move from the XSI to a 50D (or xxD of your like) as I did mine to a 40D.
The 50D is a nicer camera, to me
Let see what Canon has around the corner, I sincerely hope is not just a 50D with video