If you're getting an Assertion error popup on launching Baldur's Gate: The Original Saga, the way to fix it is by using the trusty old dgVoodoo 2 wrapper. I used the dlls in MS->x86, and used the SVGA setting in the DirectX tab. I also stretched and centered the game.

None of the other fixes found on this forums worked for me. I thought I'd use dgVoodoo 2 as a last resort, and it worked. Hope it helps somebody.