The motherboard we previously used for testing was based on the 975 chipset and offered PCI-Express slots with x16, x8 and x4 lanes. For obvious reasons, if you’ll only be using a single graphics card in your system, you’ll want to use the fastest PCI Express slot on the motherboard. In the case of our 975BX2, that would be the slot closest to the CPU, as it uses the full x16 connection.

Often you’ll only be able to tell the speed of a PCIe slot by the labelling on the motherboard. For example, the X16 slot is also marked as supporting x8 speed. This is an important bit of information for Crossfire users. After all, when two ATI graphics cards are used in a tandem configuration, the x16 connection is split into two x8 connections.

Since there are still cheap motherboards on the market that offer both AGP and PCI Express slots, you may still encounter slow slots that only support x4 speed. Tom’s Hardware ran tests in several current games at each of these speeds as well as in several resolutions. The results for x8 are a good indicator for the performance penalty that a Crossfire combination suffers, since the 975 chipset only offers two x8 connections, creating a bottleneck for both graphics cards.

In these charts, the x16 connection is considered 100 percent. Both ATI and Nvidia lose about 7 to 8 percent performance when the PEG slot only offers x8 speeds. Theoretically, the performance penalty should be twice as high for Crossfire setups. On the x4 slot, the drop in 3D speed is even more pronounced. Here, the performance of the Geforce 8800 GTS 512 dips by a full 25 percent, while ATI’s HD2900 XT loses as much as 33 percent of its 3D power.

Looking at the price of current graphics cards and the performance penalty they incur when using a slow connection, the motherboard would be the wrong place to begin saving money on. A performance hit of 25 to 33 percent is just too large.