I'm thinking of giving "Planescape: Torment" a try:

[Klorox]

How is it, really? Will I like it?

To answer that last question, I'm going to list the cRPG's I've played, and the order in which I liked them:

Baldur's Gate I
Baldur's Gate II
Star Wars: Knights of the Old Republic
Icewind Dale II
Icewind Dale I
BG: Throne of Bhaal

What do I need to know about this game before installing? What patches do I need? Anything else I should install?

TIA

[Firestormalpha]

Can't think of anything else that would need installing with the game.

Personally I found the game to be awesome.

PS:T can focus more on the dialogue portion than the combat, but it has a healthy amount of both. It pays to have a lot of points in INT and WIS, but you can play any way you want and still have a great time.

Bottomline, I say do it.

[Stratos]

The official patch is a must. PS:T is not particularely buggy but there are some annoying slowdowns in some areas.

As for the game, it's an infinity engine RPG with a unique setting, very story driven, and with a great deal to do when it comes to dialogue and quests. This is really where it's strength lie.

You'll also notice that you're stuck with a certain main character (no race, gender or appearance selection at character creation) but you can change class later. Further, equipment plays a lesser role in PS:T than in many other games. You can't, for example, equip armours, helmets or shields on the main character, or most other characters. On the upside, you can get tattoos with various effects.

[Stratos]

Whether you like it more than the games you mentioned in your post depends on what aspects of these games you like the best. If it's combat and equipment selection that gets you going, PS:T have less of that.

If it's story and character development, PS:T is the way to go.

You can talk to your party members for fun dialogue, and to learn new abilities and magic spells. This is often done in an interesting manner. One NPC will teach you spells by burning parts of your body, while another will do so by teaching you the history of his people.

[Klorox]

Which class is more fun, Fighter or Mage? I'm a big fan of both, and kinda think Mage might be better for this game (since combat isn't that important).

I know the mental stats are the most important ones, so I'll probably be starting with high INT, WIS and CHA.

[Stratos]

Mage, definately. Not only are there some interesting spells out there, but mage also relies on stats that opens up alot of the game. If you want to get the most out of the dialogues and quests in the game, a high Intelligence and Wisdom score and a moderately high Charisma score is needed. The other stats play a lesser role in PS:T, with the possible exception of Dexterity.

There are some combat in the game, particularely in the second half of the game, but that's what you've got party members for. Add to this that you're immortal in the game, you die but automatically gets resurrected at certain locations, and that you slowly regenerate wounds at a rate dependent on your Constitution.

[Stratos]

One final thing, everytime you level up you get an stat point to assign to your character so the starting character is not final.

How high your stats need to be depend on the various stat checks in the game. To get most out of the game your Wis and Int should be 20-23 and your Cha around perhaps 20. Stats in the game can be raised with tattoos and at certain locations in the game, usually as quest rewards, so you don't actually have to put points all up to those stats.

When it comes to Charisma it's a bit tricky. If you play a Mage you can use a spell that raises Charisma for a short time, and it can be cast multiple times up to the max of 25. With this you would succeed all Charisma check, but the problem is that you can only cast the spell prior to initiating dialogue so you would have to know that there is a stat check in this particular dialogue in the first place. You don't know this as the dialogue option with a stat check rarely shows up unless you have the required stat points.

[Variol (Farseer) Elmwood]

I have heard nothing but great things about it. I don't know much about it though.

Keep us posted.

[Kazilan]

Not too much more to add than whats already been said. Planescape: Torment is one of my top 5 games of all time. Don't worry too much about where you place your stats beyond some points initally in wisdom and intelligence, just build the character you want - I ended up playing through the game so many times to experience the various npcs and classes that it didn't matter too much. Constitution is also important in a number of interactions, as is dexterity. The only prerequisite for this game is that you enjoy a strong story, and can read through longer stretches of dialogue than you find in most other games. I wish more games were made in this setting.

edit - you know what, you don't even have to add a ton of points to intelligence over the other stats. It opens up additional dialogue, but as I mentioned you can utilize other stats (con and dex that I can think of, probably str too) in other types of interactions. The only one that benefits all around, in my opinion, is wisdom.