Putting current graphics cards in a PCI Express x4 slot verges on madness, incurring a performance hit of between 25 and 33 percent. Even if the motherboard in question were especially cheap, the card's price/performance ratio is affected immensely. When using an x8 connection, performance also decreases, albeit only by 7 to 8 percent.
Our switch to the X38 chipset for Crossfire tests was overdue. The dual x16 connections improve performance by 6 to 7.7 percent on average, while optimized games such as Call of Duty 4 run nearly 20 percent faster. Looking only at games and resolutions that show a tangible performance boost, we saw an improvement of 12 to 15 percent, on average.
Another thing we saw is that a dual x16 configuration is not as effective for Crossfire as a single x16 slot is for a single card. As mentioned above, a single card loses between 7 and 8 percent performance when operating in an x8 slot. Moving from an x8+x8 setup to an x16+x16 connection also only yielded a 7.7 percent frame rate increase - not the doubling we might have expected based on the single card results.
For now, the move from PCI Express 1.0a to 2.0 does not result in a performance increase with the current crop of graphics cards. Despite the fact that it doesn't feature the PCIe 2.0 interface, ATI's Radeon HD2900 XT gains two percent more performance, as do the HD3850 and HD3870, as well as Nvidia's Geforce 8800 GT. Such a small improvement could have many causes completely unrelated to the new interface, such as the newer chipset, slightly higher system memory frequency, or simply margin of error.
Whether or not upgrading to a P35 or X38 based motherboard is a worthwhile investment for a single-card system is a matter of personal preference. If you're building a new system from the ground up, we would recommend choosing an X38 board with dual x16 connections for a Crossfire setup. Upgrading a Crossfire system from an Intel 975X or P35 chipset to an X38 will only pay off if you play at resolutions of 1920x1200 or higher.
SK