Canon 50mmf2.5macro is often overlooked, you should be able to find one for $200, i used to have one its razor sharp with 1:2 macro capabilty, cheap build though which is why its inexpensive. that lens will be a great compliment to the XTi 17-55 kit lens, you dont need fast aperature for nature shots with large DOF, you should get a decent tripod though.
i have a couple pics around of senn grills taken with the 50macro to test sharpness, ill look and try to find them and post.
edit: i cant find the comparison pics of the grills i seem to have misplaced the disk, trust me that 50macro was ultra sharp to point of giving the 135f2.0L a run, this pic was taken with the 50mm with the Canon 1:1 macro converter which did softened the lens some, info below reads 70mm focal length b/c of the converter.
Camera Model Name
Canon EOS 10D
7/4/2005 6:15:07 PM
Tv( Shutter Speed )
Av( Aperture Value )
above is same image cropped, and it looks slightly front focussed
at least the ground is sharp