So it was actually the graphics card, not the mainboard, it turns out. Which means that I’ll have to start looking for a new card soon (I can borrow an older model for now- maybe Sims will run on it, maybe it won’t).
Which begs the question: should I go back to an NVIDIA GTX 650 (steady, reliable, not too fancy and newer games probably won’t run on it, and cheap to get) or should I… upgrade (with all the hassle of making the game run on a newer graphics card).