If you're getting Baldur's Gate 1, be sure to pick up the second game too as that enables you to install Tutu which greatly enhances gameplay and graphics. There's a guide for installing Tutu here: http://www.gog.com/en/forum/baldur_s_gate/newbie_guide_modding_bg1_if_you_buy_bg1_bg2_from_gog/page1

I've heard many praising Planescape: Torment and I don't think the praise is unfounded. Neverwinter Nights is fun with mods. :D

how about arcanum?

Out of that list Planescape, what should have been on your list was Arcanum.