I'm going on to ToB, and want the bonuses from choosing the evil paths in Hell. My questions are:

1) Is being evil that bad?
2) Is getting changed to evil reversable?
3) Is being evil going to screw up my relationship with Jaheira?
4) Is being evil going to make Minsc or others leave the party?

Here are the answear to your questions:

1) Is being evil that bad?

No it is not. I made a game where I was evil, and it was fun

2) Is getting changed to evil reversable?

no, it is not

3) Is being evil going to screw up my relationship with Jaheira?
It won't have any effect

4) Is being evil going to make Minsc or others leave the party?
No, all they care about is your reputation, not your aligment

Luvian got em all right except numba 2.... U CAN change it back... but not without Shadow Keeper. If ya dont have SK then some other Editing programs can probably remove the affect.

I mean ingame. Yes, you can change your aligment back by hacking your savegame, but inside the game, there is no way to change your aligment.

I thought that there was a helm of opposite allignment in SoA. Isnt there?

There was one in Baldur's Gate I, I have never found it in Baldur's Gate II.

But would it change it or not I wonder??

Yes, if you equip the helm you will go from neutral evil to neutral good.

What does the Helm do if u are neutral? Anyone know?

if ur true neutral the helm doesnt do anything, unless it has some other bonuses i dont know about