Thanks for the reply, FoxHounder!
You're a genius, sir!
I do have RGB Full set to on in my PS3, I did have the problem with darker areas. But now, following your advice, I went to Picture Options and set HDMI Black Level from Low to Normal and now it's nearly perfect!!! Although I'm far from being an expert in this field. So it looks like for gaming this is the correct setup:
Everything according to the guidelines in the first post of this thread + /Picture Options/HDMI Black Level => 'Normal'
And on PS3:
/Display Options/RGB => 'Full'
Does it look right technically speaking? Does anything else need tweaking here? Since blacks are not really deep blacks now, they are a bit "greyish", lighter than the pure black blackness.