In the simplest terms, for a gamer there are two bandwidths that need to be considered when selecting a computer. One is the bandwidth of the computer system, which includes the CPU, the memory, the buses on the motherboard, the GPU, the monitor, and all the other components. The other very important system bandwidth to consider is you! Afterall, you are the one interacting with the computer, processing what you are seeing, and providing input back into the system. To keep it simple, let’s use frames per second as our measure of bandwidth.
Do you know your personal bandwidth limitation? I do. Movie theaters show their films at 24 frames per second. Video is interlaced at approximately 30 frames per second. Once you get above about 30 frames per second, your eyes cannot process the additional information. This is a little oversimplified, but for the sake of discussion, let’s assume your personal bandwidth limitation is 30 frames per second.
At what bandwidth does your computer operate? Once again, that depends on the computer and the game. For this discussion, let’s assume a i750/P55 computer and a i920/X58 computer, both with the same components (memory, monitor, etc.) and a single graphics card. Let’s also eliminate overclocking as a variable. For a given game, which computer is faster. That’s easy. The i750 computer will beat the i920 computer every single time. Why? Because both are quad core processors operating at 2.66 GHz, but the i750 has a built in turbo mode that allows it to increase its operating speed if all cores are not being utilized. If you don’t believe me, look at this objective, independent test: http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=364...
Now, let’s change the computer systems by adding two graphics card to each computer in crossfire mode. Now the i920/X58 computer has an advantage because it has two PCI Express X16 buses available, whereas the i750/P55 computer must split its X16 bus into two X8 buses when operating with two graphics cards. Clearly, the i920/X58 computer is going to blow the i750/P55 computer out of the water with this added bandwidth. Wrong! Although you have reduced one bandwidth bottleneck in the system by using two graphics cards on X16 buses, there are many other bottlenecks in the computer that still remain, and limit overall performance. This can be verified by looking at some actual tests run at Tom’s Hardware: http://www.tomshardware.com/reviews/core-i5-gaming,2403...
I am going to summarize the Tom's Hardware article for this post using only the crossfire results, but you can read the entire article at your leisure. This test by Tom’s Hardware crossfired two 4870 graphics cards on either a i750 or i920 computer. If you have a monitor with a resolution of 1680 x 1050, the frame rates for Stalker, Resident Evil, Far Cry 2, Left for Dead, Grand Theft Auto, and Crysis were 86, 65, 170, 130, 58, and 41 respectively for the i750 computer; and 83, 66, 174, 133, 58, and 45 respectively for the i920 computer. The i920 computer was slower than the i750 by 3 frames per second in Stalker, had the same frame rate in Grand Theft Auto, and faster by 1, 3, or 4 frames per second in the remaining games. This difference in frame rates is not only small, but it is essentially meaningless because in all cases the frame rate was well above 30 frames per second, which is your personal bandwidth limitation. There is no way you can discern any difference in the performance between these two systems.
What happens if we go to a bigger monitor, one with a 2560 X 1600 resolution? For a monitor with a resolution of 2560 X 1600, the frame rates for Stalker, Resident Evil, Far Cry 2, Left for Dead, Grand Theft Auto, and Crysis were 64, 43, 140, 122, 38, and 29, respectively for the i750 computer; and 64, 45, 143, 126, 39, and 33, respectively for the i920 computer. Again, the differences in frame rates for the i920 computer compared to the i750 computer were between 0 and 4 frames per second depending on the game. This difference in frame rates is again very small because of the other bandwidth bottlenecks inherent in both computers.
Many gamers think that the i920/X58 platform is the ultimate machine for crossfiring two graphics cards because of the two X16 buses available. In reality, the difference in the performance is insignificant between the i750/P55 platform and the i920/X58 platform in most games. Is it worth paying a premium for a i920/X58 computer over a i750/P55 computer used for gaming? No! But that is only my opinion.