http://wiki.answers.com/Q/What_are_the_benefits_of_normalizing_on_steel

My understanding is that A105 as used for pressure containing parts should always be normalised and with maximum C = 0,25% (I am not a metallurgist).

ASTM A 105 is "Standard Specification for Carbon Steel Forgings for Piping Components", delivered in the untreated, annealed or normalised condition.

Heat treatment according to ASTM A105 is not mandatory except for Standard Flanges/ special flanges above Class 300 (ANSI B16.5) or of unknown working pressure-temperature rating/ condition.

Heat treatment if required must be specified and may be Annealing, Normalising, Normalising and tempering or Quenching and tempering.

This is the gist (exact wordings and clauses may be checked in the ASTM standard)

The answer provided fills your question. Regarding what is better to sustain high pressure, well, it depends. Material strengh is always a compromise between resistance and toughness. After process high-stresses in the material may lead even to a higher mechanical resistance, but will reduce toughness and material will be prone to cracking or catrastofic failure. Other factors that influence in choice is machinability, weldability, and so on. As always in materials, final answer depends on your application.

Are you sure that A105 is normalized and A105N is non-normalized material?

As short as i know, the letter "N" indicates "Normalized". I use it in my SA516 Grade 70N (which means the SA 516 Grade 70 is to be normalized).

Please correct me if i am wrong for this case. Thanks.

N stands for Normalised. I am sure about this