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So if the 770 can't utilize it why do they come with it? But I guess since the 770 can't the 760 can't also. But what what about in sli? And yea I've been watching the 280x come and go alot on Newegg. Also I don't know what cryptocoin is lol."
As Hoffa78 said, it is marketing. They can sell a product with "more power" for more money. However as I said before the 256-bit bus limits the effectiveness of the memory. Think of it in a crude example like this: The memory is a river, the speed it runs at is how fast it moves, the more it has allows more water to flow, and the memory bus is like a dam that doesn't block off the while river. A 256-bit bus will be a smaller, thinner dam, being very effective at allowing a medium amount of water (aka 2GB VRAM) to flow through quickly and efficiently, but with more water (4GB/3GB VRAM) and the same size opening(256-bit), only the same amount of water can get through as before. This means that the 256-bit bus will limit the usage of the 4GB, and while the extra VRAM wouldn't hurt, it really isn't worth the extra $50 it would cost. The 384-bit bus is like a wider opening, not really making a difference when there is medium water (2GB VRRAM), but when there is more water and amount of memory, it can really make a difference. I hope the analogy helped you!