your 18-70mm should be okay for landscapes (set between 18-20mm).
i have the sigma 10-20mm and very seldom i shoot at 10mm focal length due to distortion that i still end up setting it between 15 to 20mm.
for concerts, you will need a fast lens such as 70-200mm f/2.8 which is very expensive. your 18-70mm is short not unless lagi kang nakaupo sa harap ng stage.
i say forget the 10-20mm for the time being and either buy the sb800 or the 70-200mm lense instead : )
my 2 cents!