It should be noted that a wider bus-width will usually improve graphics when comparing identical GPUs, such as the HD4650, which is available in 64-bit and 128-bit varieties. Here are two examples:http://www.newegg.com/Product/Product.aspx?Item=N82E16814131180http://www.newegg.com/Product/Product.aspx?Item=N82E16814102829
The Sapphire card will outperform the PowerColor card simply due to the fact that it has twice the memory bandwidth, allowing the GPU to access the memory much faster. The difference in performance may not be much in some titles, but it could be rather significant in others.
So, if you're comparing two cards with the exact same core GPU in them that have different bus-widths, then the faster of the two would be the better performer. However, when comparing different GPUs, say for example the HD4850 (256-bit) and HD5770 (128-bit), the higher bus-width of the 4850 does not make it a faster card than the 5770, as the 5770's GPU is actually faster than the 4850's core.
Basically, you have to be comparing apples with apples in the first place. Get it?