I haven't yet read the books, and not sure when I will get to them. I am curious however.

The game's Geralt is a blank slate. He lost his memory, so we grow him into whatever we want him to be: fighter, mage, alchemist, defender of non-humans or a staunch pro-human, or something in the middle (when we can get away with it).

What is he like according to the official existing fluff? What choices would Sapkowski's Geralt REALLY make?

So you do want me to spoil the books or not then? Geralt is mainly a swordfighter (in the books he moves somewhat faster than ingame) but he uses signs in combat when needed. Alchemy is useful when he knows what he will be up against. I don't recall him brewing potions much though. Witcher1 intro was very true to the books in this aspect. Geralt knows he will be fighting a striga so he drinks a couple of potions that will be effective. And then he mainly uses a sword and a couple of signs.

As far as picking sides: he is a mutant and many people don't like him for it. This has been kinda glossed over in both games (aside the "mutant is pursuing me" guy). The city dwarves and elves don't mind Geralt so he gets along with them just fine. Most of the free elves and Scoiatel are arrogant assholes who don't like any humans and Geralt classifies as a human to them. So I don't think he'd side with elves that are attacking a human settlement, he'd protect the humans. But during a pogrom he'd rather side with nonhumans. He would sneak away from Foltest's service because he hates shit like that. :)

And in chapter 3 he'd go after Triss, the Northern kingdoms can plough themselves.

Geralt would most certainly go after Yennefer which is what I expect to happen in Witcher 3. He would also go after Ciri but she is not even mentioned in the game so that is a mute point. Given that Yennefer doesn't make an appearance in the game, Truss makes for a good substitute.

You'r wrong, she's mentioned many times, even in prolog...

Interesting.

How does he behave from moral standpoint? Dandelion's notes in your journal mention that Geralt is "not a good man at all" or something to that tune. However, even if you are a bastard, you can be an honorable bastard.

And is he really such a cunt-chaser as the game lets him be? In TW1, I ploughed pretty much everyone available for ploughing, but there were no significant emotional attachments in play. In TW2, I rather hesitated to do the same thus far - mostly because I was broke and it felt wasteful, but also a bit out of character (Triss is hotter and in love...)

Im not an absolute expert on the subject. But as far as your question on Geralts behaviour from a moral standpoint goes. Im pretty sure that its generally agreed that CDPR went to great lengths to make sure that Geralt as portrayed in the games is very much in line with Sapkowski's portrayal in the books.

I think that in Dandelions notes where he says Geralt is "not a good man at all" stems from the fact that the Witcher universe is morally ambiguous,there is no good or evil..just shades of grey. Plus Witchers themselves strive to remain neutral as much as possible.

Basically Geralt from a moral standpoint could be described as neutral,but more often then not he gets entangled in situations he does not want to be in. Also,if i remember correctly it has been said that Geralt is actually unusual for a Witcher in that he seems to have retained more of his human emotional senses then is usual for a Witcher after they go through their mutations. This is actually mentioned in chapter 2 of the Witcher 2 during a certain part of Roches path.

As said im no expert,but these are my opinions on the subject. Thus far ive read the english version of Blood of Elves and i personally think that CDPR are spot on with their portrayal. I think the Amnesia storyline is just a good plot device that allows us,the player,to experience the Witcher world from something of a fresh perspective.