A motherboard doesn't always come with two fully-featured PCI Express x16 slots. Available PCI Express lanes are often distributed across two slots, meaning that the second PCIe port may only run eight lanes electrically. This also means that the speed of one physical x16 PCIe port decreases when you use expand out to a second slot. If you're running a P55-based platform, like our test system, this applies to you.
But don't worry yet. Our tests show that even with the fast Radeon HD 5870 and Radeon HD 5850 cards, the performance impact when switching from PCI Express x16 to x8 is only a few frames per second (remember to check out next week's coverage for more in-depth exploration of PCI Express scaling). Additionally, both cards are throttled at x8, even if you combine a x16 and a x8 slot. According to the GPU-Z tool, the CrossFire configuration is completely synchronized.
With Nvidia, you have a bit more flexibility in slot selection. Even the fastest graphics cards are recognized as an SLI configuration, even if you don't use the SLI bridge connector and instead let the PCIe interface handle all data transfers. However, this comes at the expense of some performance. Ideally, you should always use a bridge connector to link the two cards together. Interestingly, GPU-Z shows that when you combine Nvidia cards in x16 and x8 slots, each card in the SLI configuration still runs at the speed of its respective slot.
With a bridge connector, the difference in speed is within the measuring tolerance. Without a bridge connector, though, you can clearly see that the cards use different interface speeds, and you witness a performance hit of up to 13% in SLI mode.