I would recommend a set-up simular to condyk's.... for landscapes and such, The Tokina 12-24 f4 is a great wide angle zoom when matched with 1.6crop camera bodies, it produces great color and seems to do pretty well controlling flare and such. For everyday/walk-about use... either the Sigma 24-70 f2.8 DG Macro, or the Tamron 28-75 f2.8 should do ya. I've never shot with a Sigma, so I can't comment there, but the Tamron works really well if you get a good copy. It's light for a zoom with a constant aperture of 2.8, focuses reasonably fast(the focus ring rotates as the camera focuses), but there have been a few situations where it would hunt especially in really low light. I think that's one of the reasons it cost about 1/3 the price of Canon's EF-24-70 f2.8L.
You're looking at a cost of around $880 for both the Tokina and the Tamron... this is a decent price for a set of great lenses. If you went with a Canon L set-up (EF 17-40 f4L & EF 24-70 f2.8L) it would cost around $1840. (Lens pricing based on B&Hphotovideo.com)