How to see the romance cards I have won in Witcher

There is no option to change the difficulty later in the game but you can use mods for that like: http://witcher.wikia.com/wiki/Difficulty_settings_mod

Yes, you get one of those cards for every woman you get into bed. Generally there is only one card per character, but there are two characters that have two cards each. Collecting all of them does not do anything besides letting you see them, there is no additional benefit.

These cards are probably the most-criticized aspect of The Witcher. You can try to get them all, but you can also completely ignore that part of the game. They don't have any further effect on the game.