Unless you will be gaming on a 4k monitor or triple 1080P monitors, I suggest you plan on a single good card.
Here is my canned rant on planning for dual cards:
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Dual graphics cards vs. a good single card.
a) How good do you really need to be?
A single GTX650/ti or 7770 can give you good performance at 1920 x 1200 in most games.
A single GTX660 or 7850 will give you excellent performance at 1920 x 1200 in most games.
Even 2560 x 1600 will be good with lowered detail.
A single gtx690,7990, GTX780ti or R9-290X is about as good as it gets for a single card.
Only if you are looking at triple monitor gaming, or a 4k monitor, might sli/cf will be needed.
Even that is now changing with triple monitor support on top end cards and stronger single card solutions.
b) The costs for a single card are lower.
You require a less expensive motherboard; no need for sli/cf or multiple pci-e slots.
Even a ITX motherboard will do.
Your psu costs are less.
A GTX660 needs a 430w psu, even a GTX780 only needs a 575w psu.
When you add another card to the mix, plan on adding 200w to your psu requirements.
Even the most power hungry GTX690 only needs 620w, or a 7990 needs 700w.
Case cooling becomes more of an issue with dual cards.
That means a more expensive case with more and stronger fans.
You will also look at more noise.
d) dual gpu support is dependent on the driver. Not all games can benefit from dual cards.
e) dual cards up front reduces your option to get another card for an upgrade. Not that I suggest you plan for that.
It will often be the case that replacing your current card with a newer gen card will offer a better upgrade path.
The high end Maxwell and amd 8000 or 9000 series are due the end of the year or next year.
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1. AMD requires only x4 slots to work ..... nvidia disallows SLI functionality below x8
2. You can mix RAM sized on both manufacturers but RAM usage is limited to amount on smaller card o
3. You can CF closely related AMD cards whereas nVidia will not allow this.
4. New cards can be CF'd with an APU.
5. nVidia SLI profiles are issues far more quickly than AMD CF.
6. Comparable performance cards in CF will require more PSU power than SLI
7. Comparable performance cards in CF will require case / radiator cooling
I don't recommend either CF or SLI at price points below $200 (exception being 650 Ti Boost). Systems in this budget range tend to make item nos. 1 and 4. I don't see why anyone would go forward with 2 and 3 unless a card "fell onto your lap.
Essentially, no difference.
Here is my canned rant on planning for dual cards:
a) Many won't be happy with "most games" ...even with a 760
Metro 2033 - 1920x1080 - Very High = 31 fps
Far Cry 3 - 1920x1080 - Ultra 4xAA = 38 fps
Crysis 3 - 1920x1080 - Very High 4x AA = 30 fps
Tomb Raider - 1920x1080 - Ultra 4x AA = 35 fps
b) No. Has to be considered on case by case basis. Two 560 Tis were $100 cheaper than a 580 and beat the 580 by 40%.
You won't spend $100 increasing PSU by 50 watts or adding a $10 cooling fan so not only were ya left with $80 still in ya pocket, you have 40% more performance. If you buying a 780 Ti, you are certainly not selecting a MoBo base dupon the number of x8 slots but on other features.
After running the GeForce GTX 650 Ti Boost SLI through our test suite, I have to admit that I'm impressed. The duo delivered performance easily matching and often exceeding much more expensive single-card options such as the GeForce GTX 680 and Radeon HD 7970 GHz Edition, and they don't cost as much. SLI multi-GPU scaling works well with all of our titles except for F1 2012. Scaling by going from one to two GTX 650 Ti Boost cards is around 70%, even with F1 2012 taken into account. Unlike AMD, NVIDIA does a good job of maintaining its SLI profiles, so you should be able to play new games without a long wait for multi-GPU support. However, the risk that a game will not be supported still exists, and you might, at worst, end up with single-card performance. This is in my opinion, given the massive performance-per-dollar advantage, an acceptable tradeoff. I would definitely recommend a GTX 650 Ti Boost SLI setup to a friend looking to spend as little money as possible on a high-end gaming rig.
With a combined price of $340, the graphics cards cost much less than the HD 7970 GHz Edition ($430) and the GTX 680 ($440) while still delivering comparable performance. Power draw and noise levels are slightly higher, but that's the price you'll have to pay to save over 100 bucks.
c) Never seen microstuttering in a SLI'd system where GPU cost was over $200 ... or the $150 650 Ti Boost
d) See b) above
There are no absolutes .... at the low end, I won't recommend SLI / CF. Whether SLI / CF "makes sense" will depend on cards price structure at release as well as as manufacturers response to market conditions. It is obvious however that both manufacturers have responded to the upsurge in popularity of SLI / CF over last two generations as price difference between cards have narrowed. Today, you won't find two 760s for example being $100 cheaper than a 780.