The fact that both vram modules had this effect is an indication that you are right. However, there are several machines (e.g., Quadra 950) that will only recognize 256k vram modules will accept 512k modules, but only use 256k of them.

It would be worth installing the modules at least a couple of times, as I have seen poorly seated vram cause garbled effects. Some machines seem pickier than others about how memory is seated, so you just need to indulge them sometimes.