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Dual Graphics Cards - SLI & Crossfire

If you are an avid gamer who demands the fastest frame-rate in the latest 3D games then you might consider adding two Graphics cards but your motherboard will need to support this with two PCI x16 slots. If it does then it will either work with Crossfire or SLI (Scalable Link Interface) or sometimes both depending on the motherboard. Crossfire uses two AMD/ATI Graphics Cards and SLI uses two NVIDIA Graphics Cards. It is also possible to add three or four Graphics cards if they are supported by your motherboard.

When installing Dual Graphics Cards it is preferable to have two identical cards which have the same GPU clock speed and same amount of memory. SLI generally only works if both Nvidia Graphics Cards have the same GPU, but Crossfire can usually work with two AMD/ATI Graphics Cards if they have different GPUs but are of the same series. Both Graphics Cards will only run at the speed of the slowest card.

The two Graphics Cards are connected together at the top of the card by a bridge. This allows data to be sent between both cards directly and reduces bandwidth constraints.

Two AMD/ATI Graphics cards in Crossfire configuration:

Two Graphics cards (Crossfire).
Wikimedia Commons.

The photograph above shows two AMD/ATI Radeon HD4850 Graphics cards in CrossFire configuration. It also shows the CrossFire bridge connecting the two cards together.

The benefit of having Dual Graphics Cards is the extra graphical processing power which is available when running the latest 3D games but SLI and Crossfire will not always give a performance benefit and it really depends on the application coding especially when using low resolutions. It is best to run games on a high resolution to take advantage of the extra graphical power.

It is important that if you intend to use Dual Graphics cards that you will need a Power Supply Unit (PSU) with a high +12v wattage. You can buy a PSU today that is actually designed for use with Dual or Multiple Graphics Cards.

You should also consider that having more than one Graphics Card will produce much more heat inside your computer and you may want to improve the cooling in your case with extra fans.

You can now buy a Graphics Card that has two GPUs on board which will function as a SLI or Crossfire configuration but will only need one slot. This is useful if your motherboard only has one PCIe x16 expansion port but you want more graphical power.

Motherboard PCIe x16 expansion ports.

The photograph below shows a motherboard's expansion ports (slots). It includes a standard PCI slot at the bottom along with PCI Express x1, PCI Express x4, and two PCI Express x16 slots. A modern Graphics Card uses a PCIe x16 (PCI Express x16) expansion port or slot on the motherboard.

Motherboard expansion ports.
Wikimedia Commons.

In 2003, the first development of PCIe was called PCI Express 1.0 and the PCIe x16 port had a bandwidth of 4 GB/s (gigabytes per second). Later in 2007, we had PCI Express 2.0 which doubled the bandwidth for a PCIe x16 port to 8 GB/s. PCI Express 3.0 became available in 2010 and the PCIe x16 port has a 16 GB/s bandwidth.

The specification of your motherboard will tell you if you have PCIe 1.0, PCIe 2.0, or PCIe 3.0. The Graphics Card fits into a PCIe x16 lane expansion port and for example if the specification of a motherboard states that it has PCIe 2.0 then it would call this slot PCIe 2.0 x16.

If your motherboard has two PCIe x16 expansion ports then it will most likely share the bandwidth between the two ports which means both the Graphics Cards will share the bandwidth. The amount of bandwidth it shares varies depending on the motherboard.

For an example the specification of a motherboard might state that it has 2 x PCI Express 2.0 x 16 slots (blue @ x16 mode, black @ x8 mode). This means it has two PCIe 2.0 x 16 slots suitable for two Graphics Cards and the first blue slot has x16 mode while the second black slot only x8 mode.

Generally, using two Graphics Cards in two PCI Express 2.0 x16 slots which share bandwidth so that one slot has x16 mode and the other has x8 mode will be fine for most Graphics Cards. However, if one slot has x16 mode and the other has only x4 mode then it may be preferable to use just one very powerful Graphics Card instead of installing two Graphics Cards.

You can buy a new motherboard with two PCI Express (2.0 or 3.0) x16 slots in which both slots have x16 mode however with most Graphics Cards you will not notice much more performance than a motherboard with a x16 mode and x8 mode port.

MENU (Graphics Card):
1. Graphics Card 2. Upgrade Graphics Card 3. Dual Graphics Cards
4. Overclocking