First of all, congrats on your 70-200, I have this lens as well and you will be amazed at the images it's capable of producing. You should notice quite a difference in the image quality compared to your 70-300, it's one of the sharpest zooms that you'll ever use. It's true that once you start to use "L" series lenses, it's hard to go back to anything else.


I also had the 17-40 for about 2 years and it served me very well, a nice lens for the price. If your priority is to have a wide angle landscape lens, then the 16-35 or the 17-40 would be very good choices for your XTi, you lose a little on the wide end with the 24-70 but to me, the 24-70 is probably the better walk-around lens since it has a wider range. You mentioned IS on your wish list for your next lens but yet the 3 lenses that you mentioned do not have it--how important is the IS to you?


The 16-35 and the 24-70 are 2.8 lenses, so that will give you an extra stop than the 17-40, so if your going to be hand-holding everything than the extra stop might be useful to you. Unfortunately, IS is a very good thing to have for hand held landscapes since you are not stopping action and the 2-3 stop advantage that IS gives you is greater than the one from f2.8 to f4. I personally use the 24-105 f4 with IS and love that lens as well. I'm going to be upgrading to the 5D II soon so that lens will be adequate for me to do landscapes I think. Otherwise, it's not the widest for a crop camera like the XTi.


One note of caution, I noticed that you used the word "prime" when speaking of your new 70-200 and again when you were,I believe,speaking about the "L" series of lenses. Generally, when you use the word "prime" it refers to a fixed focal length lens like the 50mm 1.4 or the 400 2.8...not trying to be critical--just helpful (even though I think most people on this forum will understand exactly what you mean).


Good luck in your search!