What game can i buy?

Maybe the witcher? I haven't got round to playing it yet but I heard it is pretty good. But out of those 2 I would say fallout if you have no friends to play with and borderlands 2 if you do have friends to play with.

Borderlands 2, AFAIK, is not as much an RPG as other roleplaying games. I think it's more like Diablo 2 in first-person perspective. And I haven't played New Vegas yet. So if you like to roleplay then pick the Fallout title. In case you like to grind and rummage in lots of random generated stuff (and you have some friends to do it with), then take Borderlands 2.

UPD: I'd recommend you some titles, but my taste is quite weird sometimes. These are Cthulhu Saves the World, Knights of the Old Republic if you like wielding lightsabers in each hand ;) and Space Pirates and Zombies if you look for arcade-ish top-down space-action games. :)

Another games I enjoyed to beat were Darkstar One (space-sim-RPG) and Leend of Grimrock (old-school first-person dungeon crawler), but I don't know whether will I run them again :)