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I have a two DVI output screencard which drives 2 monitors.

Can I use the onboard VGA to have a third monitor?

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Nope you can't.
(When it comes to integrated+external VGA mainboards, you can only operate one(s) of them. Either the MB or the external one. If you need 3 displays, then get a new VGA or add a new one.(If your motherboard supports it. Guess not. Usually these kind of MBs dont come with any kind of multiple card support but FIXME about this.))

To clear up the misunderstanding:
Usually you don't even have 2 PCI-ex slot. And even if you have, there will be SLI/Crossfire, whatever. Also, check Mainboard's manual if it works or not, usually it won't work. (These are designed for pretty basic use, not for power users.)

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Can you give me a source discussing need for motherboard compatibility with multiple discrete graphics cards? This is the first time I heard of it. Until now, I always thought that only thing needed was a slot in which you would put the card. –  AndrejaKo Jul 15 '10 at 12:12
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@Andrejako - I can't give you direct reference. I meant that way that it is not guaranteed to work. It may work, or may not work. For output, it should work. But for accelerated (like SLI), it won't really work. (Also, you can't really find 2 PCI ex slot on these..) –  Shiki Jul 15 '10 at 12:18
    
@ Shiki OK, thanks. But a simple PCI card should work for simple display, right? Also, you can use two cards without SLI/Crossfire. I also heard rumor, but never tested it myself, that you can't use more monitors than one card supports on SLI. –  AndrejaKo Jul 15 '10 at 12:32
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Crossfire/SLI are opt-in. You can most assuredly drop in a 2nd PCIe video card and run your monitors (2 per card) to extend your desktop. Yes, they'll be 3D accelerated as well, but without the performance benefits of SLI/Crossfire. And yes, SLI/Crossfire is 2 monitors only. No support needed other than slots. AndreajaKo: Radeon 57xx, 58xx, maybe 56xx do "Eyefinity", which is 3-6 monitors on a single card, though some need to be Displayport. You can get around that requirement with DP converters (DP-to-VGA being the cheapest bet). –  phresus Jul 15 '10 at 12:48
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There are also USB video adapters. While I wouldn't watch a video on them, they are usable for most other tasks. –  steve.lippert Jul 15 '10 at 14:48
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I'd like to add here that some computers do allow you to use both onboard VGA and a graphics card (whether it be PCI/AGP/PCI-x, etc).

Also, some motherboards let you enable / disable that feature in the bios.

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Right, the AMD motherboard in my computer supports ATI Crossfire, so it will work simultaneously with an add-in card as long as that card is also ATI. –  Brian Jul 15 '10 at 13:36
    
I was able to get mine working by telling it to use the onboard as the main video card. If it is set to the PCI-e card (the default), it didn't work. If you've got the hardware, give it a shot. –  Darcy Casselman Jul 15 '10 at 14:24
    
@Brian -- I understand, but I am not only talking about crossfire mode. I meant that it is possible to use different video cards along with onboard gpu. –  MisterITGuy Jul 20 '10 at 19:22
    
(But, for SLI / Crossfire, the same chipset is required). –  MisterITGuy Jul 20 '10 at 19:22
    
I had to select the on-board video as the default. On boot it recognized the other video and windows integrates all of them together. –  steampowered Nov 18 '11 at 12:20
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I've used both on board and dedicated at the same time. Not all motherboards support it. If you have more than one PCI slot, or more than one PCI-x slot, you should be able to add multiple cards and run monitors on all of them. Crossfire / SLI does't have to be enabled. I've had 2 PCI-E cards in my computer for a while. They are SLI compatible but I just use them as standalone cards.

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