FPS help (Condemned 2 and Dark sector related)

[TheTenth]

I'm just bad at FPS ... I nearly finished Medal of honor 1 on PS1, did 2/3 of Killzone until most guys throw grenades and kill you in one time ...
Not being familiar with these games : do you have to do and re-do and re-do a stage to know exactly where opponents are and move? do the weapons getting better help you survive? do you have to play "commando"? (= stay hidden, shoot a guy very fast and then hide again?)

I did the Condemned 2 demo, it seemed different to me as melee is an important aspect (parry, counter, combos), but there's some shooting it seems. Is there a lot of enemies at the same time? or just 2 or 3 you have to be intelligent to get into contact to kill them? It seems you can deactivate the reticle in the options, how do you aim then?

I'm interested in Dark Sector too ... i heard it plays like Gears of War, but for me Gears of War is a FPS where you see part of the character but you still look only the center of screen = reticle to aim at enemies ... or is there no reticle and only guessing the trajectory like in Gauntlet, Dark alliance, Darkstone, Untold legends? I'm quite good at these last games.