I am a first time buyer of graphics card. I have made up my mind on 4770 Sapphire reference card. Comparing the specs of 4770 Vs 4850 how much will bus width matter. I mean to ask does bus width help higher resolution or higher settings (as in high details Vs Medium details OR no AA vs 4xAA).
It depends really, although in general performance, its a push.
A higher Bus width allows for the transfer of more data per LOAD operation. In theory, a 256-bit GPU can access 4x as much data as a 64-bit CPU in a single LOAD operation (hence, why CPU's tend to be clocked higher, to allow for more LOAD operations to compensate for a small data bus).
Provided the GPU can execute all its data in a single cycle, the higher bus width would clearly be faster. Of course, that almost never happens, so most of the data needs to be stored in RAM, where faster RAM speeds then start to close the distance.
Hence theres almost no diffrence between the two approaches, at least in current applications. I suspect that purley mathematical functions, as opposed to rendering, would run much faster on a wider bus though (maybe the reason ATI refuses to implement Physx? Because NVIDIA's larger Bus would be too big an advantage in benchies? )
The bottom line when comparing those 2 cards is that the HD4770 is 5-10% slower than the HD4850 overall. The HD4770's advantage over the HD4850 is that its processor is built using a 40nm process which allows for greater power efficiency. Which you should get depends on how you personally value those 2 aspects and price.