I think it's great that you have a mom who likes to play BG/IWD etc. Heck, I'd like to find a girl who is like that, but closer to my age (40ish)!!! I too have played and enjoyed both the BG and IWD series... I'm in the later stages of IWD2 playing cooperatively online with 3 local friends. Next I'm going to play the NWN sequel - Shadows of Undrentied.
I haven't played PS:Torment, but I've heard alot of people raving about it. The only thing is I've heard a few women complain that the protagonist is a male, and they wish they could play a female protagonist. I can understand that. Never-the-less, they still liked the game.
BTW, if she has a "nice Dell", I'm guessing it could handle NWN ok. NWN would be right up her ally - and even tho it was made with multiplayer in mind, it plays like BG/IWD - that is the story is the same, and you can play through it cooperatively on line or by yourself. You cannot have a party of 6 in single player tho... rather you play as a single character and can have one henchman/henchwoman

join you for a party of 2. You have limited control over your henchman.
NWN plays pretty well on my PCk, which is a 1.3 ghz using the Radeon 32DDR video card. I'm guessing if your mom's system is near that or better, it will work fine. Heck, I read the box of Unreal II and my system meets the recommende specs for most of the items and it is supposed to be a hardware hog. I think the only thing I don't have which is recommended is 64 mb video memory.