Here is my suggestion, feel free to skip Unfinished Business (step 6) if you want to go for a purer, less modified experience. After all this is done you should have a stabler game running at a higher resolution.

1. Install Baldur's Gate II
2. Download Baldur's Gate II Fixpack (Download links are on the left side) and extract it to your Baldur's Gate II directory.
3. Run the newly extracted setup-bg2fixpack.exe (if not automatically started).
4. It is safe to select yes for all the optional fixes, but if you want to learn about each in detail, consult the readme file that comes with the fixpack.
5. OPTIONAL: Download and install BG 2 Unfinished Business. This mod restores things that were previously cut from the released product, so while you don't need it, and it does add new content, it's pretty nice to have, especially if this is not your first time playing.
6. Download the widescreen mod and apply it in the same manner as the fixpack. This makes it possible to play the game in high resolutions and in widescreen format.
7. OPTIONAL: (for more experienced users) Download Baldur's Gate II Tweak Pack and apply it in the same way as the fixpack. Consult the readme file for details of each tweak - too many to list here. If you don't know what a tweak does even after consulting the readme, just skip it.

Oh by the way, you should have posted your question here: http://www.gog.com/en/forum/baldur_s_gate/page1