I have 2 lenses in my mind for my D300. 24/70 2.8 or 17/55 2.8. Does the DX give same advantages compared with the 24/70?? Please help me to choose one. Most of my pictures are landscape and faces

Hm... well, the 24-70mm is overall the sharper lens wide open. I'd also say it has less distortion, vignetting, and flare.

But 24mm just isn't as wide as 17mm, so if you need that angle of view there is no substitute.
I don't feel the 17-55 DX is necessarily the best choice for landscapes, though either lens is versatile enough for both of your intended subjects.

Hm... well, the 24-70mm is overall the sharper lens wide open. I'd also say it has less distortion, vignetting, and flare.

But 24mm just isn't as wide as 17mm, so if you need that angle of view there is no substitute.
I don't feel the 17-55 DX is necessarily the best choice for landscapes, though either lens is versatile enough for both of your intended subjects.

As a walkaround I would highly suggest the 17-55 f/2.8 just because the wide angle you get at 17mm vs. 24mm. makes it a much better starting point for landscape photography. The issue is that wide open you're not getting the same sharpness (f/2.8 is useful for portraits so keep this in mind) and it has tons of elements inside making it very prone to flare (like my Sigma 15-30). I used to own the 17-35mm and it was a nice zoom range but the 17-55 is perfect for what you are describing. I would suggest it for your purposes over the 24-70mm.