The 40D has a pentaprism viewfinder so it will be much brighter, it goes to 3200 ISO, it is a fair amount larger and has a more solid build, it has a max 1/8000 shutter speed and a 1/250 flash sync, almost 3x the amount of RAW buffer, the rear dial and top LCD screen. It also continues to use the same size cards as what you currently have. And is rated at almost 2x the number of shots per charge on a battery.
However the XSi does give you the big thing I was looking for when I orignially upgraded from my XT in spot metering.
I love my 40D and am glad that I purchased it, after almost 10K shots with it, I picked up my XT the other day and it really felt like a toy in my hands, I didn't notice it when that was what I was using all the time, but now I really do.