Figuring out the relative strengths and weaknesses of graphics cards isn't easy. Besides the graphical processing unit itself, you have to consider memory, stream processors and other components. Then there's the data transfer rate, or "bitrate," of the bus that links the card's memory to its processor. Once you take all of these into account, though it may seem counterintuitive, a 128-bit graphics card isn't necessarily better than its 256-bit counterpart.
Function
The bitrate of a graphics card refers to the amount of data the card is able to move between the GPU and the RAM with each clock cycle. This is one component of the card's overall memory throughput, or bandwidth. More bandwidth allows the card to draw to the screen more quickly and with better resolution, resulting in smoother and higher-quality images.
Performance
A graphics card's actual throughput or bandwidth, measured in gigabytes per second rather than bits, depends on a combination of the bitrate of its bus and the frequency of its random-access memory. This is calculated by dividing the bitrate by 8 to convert it to bytes, then multiplying the result by the RAM's frequency in megahertz. For example, a 128-bit graphics card with 3,000-MHz RAM has 48 GB/s of bandwidth. All other things being equal, a 256-bit card has double that amount, or 96 GB/s.
Misconceptions
While it's true that, all things being equal, a 256-bit graphics card offers double the memory bandwidth of its 128-bit counterpart, realistically, two graphics cards won't be separated only by bus size. Other factors, lsuch as the amount and speed of RAM, are always at play. For this reason, cards should always be compared by their overall memory bandwidth, never their bitrate.
Uses
Both 128- and 256-bit graphics cards are suitable for the vast majority of users. They are more than adequate for basic computing, and also for budget gaming and general use of 3D graphics packages like 3ds Max and Maya. Enthusiast-level cards remain 256-bit, with only the most powerful and expensive performance-level cards rated at 320-, 384-, and in rare cases, 512-bit.
Overclocking
The bitrate of a graphics card is physically limited by the number of pins the processor has, so it can't be overclocked like a processor. A 128-bit card can never, in other words, be increased until it becomes a 256-bit card. Because memory bandwidth is a product of bitrate and RAM frequency, however, it's possible to increase a card's overall throughput by overclocking its RAM. And because bitrate and RAM frequency serve no purpose other than to contribute to overall memory bandwidth, any increase to one is effectively an increase for the other. That said, overclocking RAM is always more effective on cards with higher bitrates. An increase of 128 MHz to a 128-bit card's RAM frequency results in an additional 2.048 GB/s of throughput. If the card is 256-bit instead, that gain doubles to 4.096 GB/s.