Looking for games where you have to take decisions...

The game of Life... Real Life. You don't even need a PC for it. :)

Vampire: The Masquerade - Bloodlines

One of THE best RPG's I have ever played. It's a bit on the old side though, so graphics go a bit haywire in relation to the camera (ie, your camera goes through things A LOT).

Fallout new vegas C:

Bioshock & Bioshock 2 (ish)
Deus Ex (original, not sure about HR, haven't got to it yet :( )
Mass Effect series (I've heard)

Oh...and S.T.A.L.K.E.R. too, but I haven't got to the end yet :)

The Walking Dead by Telltale. Awesome game.

You should really get into Visual Novels, if it has to be all about making decisions. One of the best are such ones as Fate/stay night, Kara no Shoujo, Higurashi no Naku Koro ni, Tsukihime, Umineko no Naku Koro ni, Ever17 -the out of infinity-, Chaos;Head, G-Senjou no Maou and many more. Get onto some forum oriented about them and you will learn everything you need to know.

king arthur has some choices. it's a "copy" of total war with fantasy units and a few RPG elements. normaly each storyline has 2 outcomes, a few quests even have 3 or 4 outcomes. i havent finish the game so i don't know how meanigfull the choices are.
fable series has choices but number 2 and 3 are very easy
dawn of war 2 has choices. especially the chaos expansion.
warlords battlecry 3 has choices.

ground control, nevrewinter nights and codename panzer phase1/2. i think these games have choices but it's been a while since i played them so i can't really remember

To be perfectly honest, Heroes 3 is the only game where each session is random and your decisions make or break the result. Besides, the game just pwns every other game:)

Fable: The Lost Chapters. Ignore the sequels.

Life! Its in beta still though... It can be a real bitch to set up and if you start losing things, you might be fucked.

The witcher 1&2, dragon age 1&2 and mass effect 1,2&3

"Already played ... Heavy Rain."

I'm so sorry

The inFAMOUS series.
Dear God.
The music, the important choices when it comes to the second game, just everything, is lesex.
You'll lose out unless you play it.

Fallout 3 + Broken Steel (DLC). Great game with soo much decision, you have the power to decide how is your character going to be, your allies, change the history and the ending, etc, etc... Give it a try, you will probsbly enjoy it! Btw, you should also check Skyrim.

Dragon Age: Origins ?:P

maybe Star Wars: Knights of the Old Republic.

A lot of good games have already been mentioned, so I'll add another, probably a bit obscure one - Shadow of Memories (also known as Shadow of Destiny).

It's an adventure game with multiple endings and a really good story, although the gameplay is nothing special and the whole game is a bit slow-paced. However, it is definitely a unique and fun game. Nowadays, it might be hard to find, though.

Fahrenheit, also known as Indigo Prophecy

Pretty much everything I came in here to post has already been named: the Bioware catalog, Alpha Protocol, V:TM Bloodlines, etc.

One that hasn't been mentioned that might or might not fit is Bastion. There are only a few decisions to make with regard to the story, but they're very, very well done.

if you're into hardcore roguelikes(even though this one has graphics and great community-i'm not a roguelike fan usually and I really like it), than ToME 4 will suit you: http://te4.org

The sims

Nothing have done it better than the Witcher 1 and 2.

Especially two, your whole second ACT can be different based on your choices, and many other stuff.

Mass Effect Trilogy.

But don't play the last part until the end before the new dlc is released!!!

Try the "Age Of" series. My favorite is Age Of Mythology.
They are from the RTS genre, so you're constantly making decisions. :)