I'm new at this whole thing, so my way of telling you will lack some knowledge, but...
I am part of three friends who have an XT, XTi, and XSi. I have used all 3 of our cameras (although, obviously I've used my own XT the most). I will say the larger viewfinder is very nice, but I also wanted to throw this out there... There is something about the XTi (and I'm assuming XSi) that makes it autofocus better in the dark. My friend and I were shooting side-by-side, aiming at the same spot, same setting, and mine was hit or miss. I ended up switching it to manual focus just so I could get the shot (bad idea... my shots came out blurry!)
I bought the XT just b/c I'm on a tight budget and needed something quickly (I'm going to Africa in a month) and as cheap as possible, but I definitely would upgrade to the XTi, XSi, or 30D if I had the money for it. As it is, it's like buying a puppy... you think the initial investment is the big part of the purchase, but it's all the "extras" that keep adding up! (I already discovered last night that I need another lens to cover the 55mm-75mm range!)
ETA: I also have a friend who has a 20D, 30D, and 40D but he hasn't let me get my hands on them, yet.