I personally wouldn't shoot any farther than about 100yds with iron sights. I just don't think I'm that good! Your distance may vary, but under no circumstances would I shoot any farther away than I can positively, beyond doubt identify what I'm shooting at. Again, for me, that's about 100 yards sans scope.
The primary difference between a good scope and a great scope is clarity of image at any magnification and the amount of light it lets in. Pricier scopes use better glass - less distortion, more light, which is especially important at dawn and dusk, or any other periods of low light. Objective size matters not; quality lenses are key. An expensive (Zeiss, etc.) 40mm scope will see better than a 50mm Tasco. For top quality at a reasonable price my two scope choices are the 3-9x40mm Nikon Buckmasters and the 2.5-10x50mm Nikon Monarch. I don't personally need any more magnification that these scopes provide. With these scopes, shots to 400yds and maybe beyond become a real possibility for me!