If you shoot landscape, you'd want an ultra-wide lens. Canon EF-S 10-22 or Sigma 10-20 are very good choices. The Tokina 11-16 is very sharp and 2.8 but I found that the 10-22's 22mm on the long end to be very useful.
If you want a longer tele, I'd go with either the 70-300IS or the 70-200L, or sell the 18-55IS and get the 18-200IS, I have the sigma 18-200 OS and and the range is incredible.
Last but not least... actually IMO should be first: get a good flash, 430exII or 580exII. I wish I got it 2 years ago, because 90% of my pics are taken indoors.
Fast prime are nice but the DOF is too shallow wide open, IS is nice too but if your subject is moving it can't help, and if you shoot in raw, you can use Noise Ninja to clean the noise quite nicely up to iso 800.
But, all of the above don't come close to my 580exII.
I take shots at iso 200, 1/250sec with my slow zoom lens stopped down to f5.6 or even f8 and get fantastic clean well exposed results.
Don't take the xti's built-in flash as an example, it is as useless as a point of shoot flash, and in the two years I used the xti i used it maybe 3 times.. the 40d's flash is an improvement but I was literally blown away with the results I get with real flash.
Cheers,
Erez
