I pulled this from a review on Tom's Hardware.
http://www.tomshardware.com/2008/01/23/crossfire_meets_pci_express/page5.html____________________________________________________________________________________________________________
Going From PCI Express 1.0a To 2.0
To get right to the point, upgrading from PCI Express 1.x to 2.0 is not worthwhile right now. The current crop of graphics cards just doesn't tax the PCI Express bus enough for a difference to be visible. All test cards, across the board, showed a minor performance boost of 1 to 2 percent, with the HD2900 XT reaching better results despite the fact that it doesn't have a 2.0 interface. This improvement may be caused by the newer chipset or the higher memory speed. Also, a difference of 1 to 3 percent is also slim enough to be considered within the margin of error for testing.
Single-Card Performance - HD3850 OC (256 MB)
PCIe 1.0a x16 vs 2.0 x16 fps Percent
HD3850 (256 MB) OC PCIe 1.0a x16 1576.5 100.0
HD3850 (256 MB) OC PCIe 2.0 x16 1612.4 102.3
Single-Card Performance - HD3870 OC (512 MB)
PCIe 1.0a x16 vs 2.0 x16 fps Percent
HD3870 (512 MB) OC PCIe 1.0a x16 1795.3 100.0
HD3870 (512 MB) OC PCIe 2.0 x16 1828.6 101.9
Single-Card Performance - HD2900 XT (512 MB)
PCIe 1.0a x16 vs 2.0 x16 fps Percent
HD2900 XT (512 MB) PCIe 1.0a x16 1796.8 100.0
HD2900 XT (512 MB) PCIe 2.0 x16 1832.8 102.0
Single-Card Performance - 8800 GT (512 MB)
PCIe 1.0a x16 vs 2.0 x16 fps Percent
8800 GT (512 MB) OC PCIe 1.0a x16 2137.9 100.0
8800 GT (512 MB) OC PCIe 2.0 x16 2185.1 102.2
Single-Card Performance - 8800 GTS (512 MB)
PCIe 1.0a x16 vs 2.0 x16 fps Percent
8800 GTS (512 MB) OC PCIe 1.0a x16 2235.3 100.0
8800 GTS (512 MB) OC PCIe 2.0 x16 2260.6 101.1
It remains to be seen what the situation will be when newer graphics cards such as the Geforce 9 or the Radeon 4xx0 begin transferring larger amounts of data over the bus. At any rate, the P35 and 975 chipsets are easily sufficient for the current generation of cards.
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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.
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The these Higher end cards we see just a minute increase in performance.
I think some of it has to do with the Memory interface. Once we see greater interfaces like the GTX's 386 bit then I think we'll see a better performance jump using the 2.0 express slots. But I'm just assuming.
