This is the ultimate collection of one of the finest games ever made. Baldur's Gate is part 1 of the trilogy, while BG2 and its expansion form parts 2 and 3.
Gameplay: this is the best existing incarnation of Dungeons and Dragons rules. Other games using D&D rules are either focused only on combat (Icewind Dale, Temple of Elemental Evil), far too restrictive on party size (Neverwinter Nights), or require an insanely high-end machine (Neverwinter Nights 2).
Storyline: astonishing, superb, extraordinary. It just doesn't get any better. Every character is rounded; every storyline is fulfilling; you have complete freedom while at the same time having a very definite agenda; combat is always purposeful rather than just mindless hacking-n-slashing. The game is bursting with atmosphere, wit, and challenges.
Graphics and sound: BG has a top-down 2D view, but with gorgeous, breathtaking hand-crafted environments. If you insist on 3D, this isn't the game for you; play Oblivion instead. ;-) Optimal screen resolution is 1024×768 for BG2 (you can't change resolution in BG1). Some of the voice-acting is top-notch. The music is very good – the combat music, in particular, is very fine indeed.
A suggestion: you may want to think about using the “Tutu” mod for playing BG1: this basically imports all game content from BG1 into the BG2 game engine, which allows you to make use of various options (character classes, screen resolution, 3D effects, etc.). On the other hand, you may decide that it makes BG1 a bit too easy.
This game is designed for an immersive, long-term experience, with 80–300 hours of gameplay. If you want something you can play for five minutes between phone calls, this isn't for you. Basically, this is the most lovingly-crafted game I have ever encountered; and this is the best edition of it.