Power supply. A dual configuration setup usually will consume more power than a single configuration. So power supply will limit what dual gpus you can get.
There are a couple of ways to approach selecting a graphics card setup:
1) Look at the level of performance that you want and then identify setups that meet that level of performance (typically it's significantly less expensive to achieve the same level of performance as a higher end single card).
2) Identify your budget and buy the most powerful single gpu card that you can get. Reduces the chances of any multi-card related issues.
3) Identify your budget and buy the most powerful pair of cards you can get for the money. Multi-GPU setups aren't extremely exotic or finicky, but they do increase the likelihood of enountering some kind of issues at some point. How great is that likelihood? Hard to put a number to it.
With all setups ensure that your system meets the minimum required specs. As bear mentioned, ensure your power supply is sufficient. Also, if you go multi-card (Crossfire or SLI) make sure your system supports it.
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General guidelines? Well the first is if you have a multi-GPU capable board, second is if your PSU can handle it, third is do you have ventilation in your case for it. Priority 0 is of course budget . Anyway, it's generally best to start off with a single GPU and add another one latter for more performance. There are always some issues with multi-GPU configurations with new as well as lesser known games. Skrim is one of the more recent examples where Crossfire Users had to wait for updated drivers to properly run Skyrim.
Personally I don't mind waiting for a bit after launch to make sure that Crossfire will work with the game, or at least that I know what I'm getting into. In the rare game that I do disable crossfire (looking at you Force Unleashed) a single 5870 is usually enough that it doesn't bother me.
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I can see by the responses I didn't ask my question to get the response I was looking for. Let me try it another way. What applications/configuration/games would lead me to look at having dual GPUs instead of one?
It's best to do some extensive research and find out what you'll be using your computer for.
I know its a lot of muckering around but thats the only way you'll be satisified with the choice you made.
If its mainly gaming (not sure if sli/crossfire is only meant for games) then you'll have to ask yourself.
Sli vs Crossfire - Nvidia vs Amd. Which brand and version of their cards perform well in a dual configuration and should I care about micro stuttering in certain games I might play?
Games - Will my games, new or old have any issues with dual configurations?
Resolution/multiple monitors - Do I need multiple monitors? What resolution will I be using?
Vram - how much memory do I need? More the better but depends on the game and the level of detail you'll be/wanting to play. In a dual card configuration the memory does not double. If the two cards have 1gb each it will stay as 1gb.
Future proofing - Will I benefit in the long run? Depending on your upgrade cycle, if you're one to hang on to your system for quite awhile then dual most likely will be an viable option to keep your computer competitive.
Sli vs crossifre vs single - most common question
If you're fussy, want no hassles and want smooth looking performance then a single card is best for you. You’ll need to way up price/performance/hassle ratio to figure out if you want dual or not.
Message edited by boju on 01-21-2012 at 06:24:16 AM
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I can see by the responses I didn't ask my question to get the response I was looking for. Let me try it another way. What applications/configuration/games would lead me to look at having dual GPUs instead of one?
Thanks for your patience.
BTW, I thought the OP was clear in what you were asking.
Applications - none I can think of
Configurations - Nvidia 3d surround requires 3 of the same digital outputs to power 3 monitors and the only card that I can think of that has 3 dvi ports is the gtx 590 which is the fastest and most expensive card they make. If you don't have a 590 then you would need to use, for example, sli gtx 460's since each card has two dvi ports.
Games - This is a very broad topic. Let's just say you have a gtx 560 and you play battlefield 3. At best you are probably only going to be able to play in med/high graphic settings at a resolution of 1920x1080. If you attempt to increase the resolution or graphic quality, your frame rates will drop and depending on how low they get you will have very noticeable lag in the display and the game may not be playable. The solution is to get another 560 and sli it with the one you already have and the two cards will work together to give you more power to increase the graphics quality and still maintain a high enough frame rate to play the game.
This really depends on the game you play, the resolution, the graphics quality to determine what card(s) would be nescessary. Are you just wondering why someone would want to have two gpu's or is their a reason behind the curiosity?