The Star Wars: Knights of the Old Republic games are pretty good. Well, I've only played the first on the Xbox (absolutely amazing), but the second game's supposed to be better in some ways. Pretty good dialogue trees. They make them both on the computer.
Neverwinter Nights is very good, and you can mod it to death. Some cool dialogue trees, and an interesting character creator. Not as open-ended as Arcanum, from what I've played, but as I said, it's extremely mod-able.
Morrowind or Oblivion are pretty good choices, but not exactly deep with conversations. And with 3rd person, it's a little loose, however, you can mod those pretty well. From what remember, they aren't too great with dialogue trees. So you'll rarely find an occasion to lash out at someone.
Come to think of it, some of the people that made this game made an earlier PC RPG epic - Fallout. The first one. It's most likely more outdated graphic-wise, but supposed to be really great.
Oh yeah! Mass Effect. The same people that made Neverwinter Nights and the first Knights of the Old Stuff game. They made some giant sci-fi epic for the 360. 3rd person, and (supposedly) deeper than any other game when it comes to dialogue options (trees). Heard it's pretty glitchy though, and the weapons and item systems are a little weird, but a cool part is that they're making this series a trilogy. The game carries over save files into the sequential games, and in a game like this - where just about everything you say and do reflects things in the future - that's pretty amazing. But again, not for the computer.
Damn I've said "dialogue trees" too many times for one day.
Neverwinter Nights 2, Baldurs Gate series (pc ones), Knights of the old republic 1 and 2, jade empire, temple of elemental evil, fallout 1 and 2, icewind dale series, vampire the masquerade: bloodlines, planescape torment...