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I have an SLI ready power supply and video card. I'm looking at getting another video card so I can have 3 monitors. What is SLI? I think it's got something to do with treating 2 video cards as 1. If that's the case, is there any performance hit for using 2 video cards of different memory/specs ?

I'm running GeForce GTX 460

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You can probably add a second video card without sli. They do need to have the same driver if its an older version of windows. Sli isn't needed for adding another output –  Journeyman Geek Mar 8 '13 at 4:17
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From wikipedia

Scalable Link Interface (SLI) is a brand name for a multi-GPU solution developed by NVIDIA for linking two or more video cards together to produce a single output. SLI is an application of parallel processing for computer graphics, meant to increase the processing power available for graphics.

An SLI ready card or power supply means it's capable of adding a second (or more) video card. Adding a second card usually can increase overall performance but there are times when the communication between the two cards can create overhead (see micro stuttering)

You don't specify which video card you currently have but you may not need a second card just to drive three monitors. If you just want to have an extended desktop with three screens your current card may support that option. Depends on the outputs and options. If you want to have three screens for game playing you may need a second card depending on what resolution and level of quality you enable but this is not a requirement. I use a single AMD XFX 6950 on three monitors for both work and gaming just fine.

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I have 3 monitors connected, and it keeps "disconnecting" the 3rd monitor. It auto selects "disabled" from the dropdown list and when I choose to extend the screen it just disables my other non-primary monitor –  Webnet Mar 8 '13 at 3:23
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http://forums.bit-tech.net/showthread.php?t=198664 seems to indicate that your card cannot run with 3 monitors enabled. You could either upgrade to a card supporting 3 monitors, or link a second card through SLI if your motherboard has a second slot for a graphics card. Bear in mind that two graphics cards generally use more power than 1 alone. –  washbow Mar 8 '13 at 3:33
    
Does my motherboard have to be SLI compatible? It looks like it's not... newegg.com/Product/Product.aspx?Item=N82E16813131646 and there's a motherboards section on nvidia SLI hardware :-\ geforce.com/hardware/motherboards –  Webnet Mar 8 '13 at 4:59
    
Yeah a motherboard based on AMD chipset is not likely to be SLI ready (since NVidia is a main competitor). It is Crossfire ready. –  Brad Patton Mar 8 '13 at 14:11
    
@Webnet Some GPUs have e.g. 3 outputs but can only use 2 at the same time, check the specifications of your GPU. From what a quick search told me: A single GeForce GTX 460 does not support 3 simultaneous monitors. –  Jochem Kuijpers Jan 13 at 22:43
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SLI ready means that the video card can be put into a SLI configuration. SLI configurations essentially "connect" the video cards together and allow them to work together. There is a performance hit when adding a second video card. When adding a second video card you should expect to see about a 90% increase. If this was a perfect world it would be a 100% increase but sadly this is not a perfect world. Memory shouldn't take a hit in terms of size however.

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Is the performance decrease only applicable if they are in SLI config? Is the SLI config a configuration or physical connection between the two cards? –  Webnet Mar 8 '13 at 3:04
    
A physical connection and a configuration and when I say performance decrease you won't actually lose speed you will gain speed it just won't be twice as fast but it will be close. Now I am not positive but I am fairly sure you can run two cards without putting them in SLI. –  Griffin Mar 8 '13 at 3:06
    
Is it "better" to put them in SLI? –  Webnet Mar 8 '13 at 3:09
    
Depends on what your doing. However most people feel putting then in SLI is the best way to go. Also you have to make sure the two cards are compatible. While most cards can do SLI / Crossfire not all cards work together. They need to same GPU or they are useless. –  Griffin Mar 8 '13 at 3:11
    
Explain performance hit. Bench marks show and improvment to FPS in SLI multi card set up. –  Carl B Mar 8 '13 at 3:32
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