For the most part i can agree with that statement but is a 1 gb 256 bit hd 6850 twice as fast as a 1gb 128bit 7770 ?
Memory/clock speed can somewhat overcome the smaller bus.
No no no, I meant that if everything else is the same THEN 256-bit is twice as fast as 128-bit. You know, perfect environment stuff A good example is the GTX 460 256-bit vs. the 192-bit version. 25% difference, if the clocks are the same.
Definitely 2GB, but that doesnt mean you need to sacrifice down to an 128 bit bus. There are 256 and higher on most cards. The memory used has to deal with resolution, but more importantly, the texture sizes. As games get higher and higher quality textures, the memory used will sky rocket. textures at 1920x1080 are 4 times smaller than 3840x2160. As people can buy 2160p monitors now, its only a matter of time before games will be created with textures that take advantage of this. Ive heard of some people seeing usage of 1.5GB already with one monitor. Get 2 2160p monitors, and games using textures of that resolution, and you would be seeing usage of 12GB. But I dont see this happening for another 5-8 years haha.
What resolution are you using? In general, I would think that a 128-bit bus is going to bottleneck 2 Gb of memory...and 256-bit is definitely going to have more throughput. A lot of whether this makes a difference, and what is faster than the other, depends on the cards though. No one thing is the deciding factor. For example, an old 8800 GTS has a 192-bit bus, but will be out performed by a newer card with only a 128-bit bus. What are the cards you are thinking of?
128bit at 2ghz is the same memory bandwidth as 256bit at 1ghz. but when it comes to graphics cards, a card can have lower memory bandwidth and still have better performance, because there are other factors, gpu IPC, stream processor count, ROP count, texture units etc. The only real way to tell is to read reviews of the card you want to buy.
It depends on the card... If its a low end card it won't be able to use the 2gb of ram no matter what the memory bit is - and it being a 128-bit means it is a low end card so the 2gb is totally pointless. That being said the only way the 1gb 256-bit card would have issues is if it was a high end card, but high end cards don't have 1gb anymore so i assume its a low-mid range so 1gb is plenty(any more ram would be pointless because the card would not be powerful enough to use it anyway). SO based off the little information you gave us I would assume the 1 gb 256-bit card is the better of the two.