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I wonder how 2x crossfired ATI graphic cards could be compared with a single card with double capabilities? For instance, my video card (ATI 5570) has 1GB memory (128-bit). If I crossfire it with a second card, will it out-perform a card with 2GB memory which is 256-bit? Which one gives more performance?

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Eeeeeuuuugh. Hard to know. From specs alone, graphics cards are hard to compare, if possible at all. Normally, if you want to know how good a card is, the only reliable way is to look at benchmarks and reviews. So unless someone has ever tried that kind of test, you'll never know, I say. Besides, CrossFire requires identical cards, I believe, and the cards need to be enabled for that, no? Is it the case with yours? And also, I think I read somewhere that not all applications/games are able to use dual graphics cards fully. –  Ariane Jan 13 '13 at 18:52

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For instance, my video card (ATI 5570) has 1GB memory (128-bit). If I crossfire it with a second card, will it out-perform a card with 2GB memory which is 256-bit?

No. You will still have 1GB usable memory, with 128 bit width.

(The same is true both for AMD cards in crossfire and Nvidia cards in SLI).

Most programs would only use the primary card, and run at the speed of that single card. The second card only comes into use when you have a game which supports crossfire or SLI. Then all the data stored in the first cards memory is also stored in the memory of the second card, and the second GPU takes over part of the calculations.

This means that you would have 2GB memory, but only as two 1GB copies of each other. Same with crossfire with 3 or 4 cards.

Which one gives more performance?

A single card with double the performance is almost always better.

The table below should be read with care, since the result vary from game to game, but:

Single card (say 100% base speed)   ---------->   100% performance
Single card (say 200% base speed)   ---------->   200% performance

Two cards (each card at 100% base speed)   --->   180% performance

Three cards (each card at 100% base speed) --->   250% performance

...

Etc etc. The overhead of extra cards causes diminishing returns.

The only reason to use more than one card in Crossfire or SLI is if you happen to already have multiple of those cards, or if you need absolute maximum performance. (And then you crossfire multiple high performance cards).

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OK. Thanks. I get it now. Just one question, considering the lower cost of my old graphic card, wouldn't it be better if I bought one just like it and corssfired? I want to know which one gives more performance for same money? –  Alireza Noori Jan 13 '13 at 20:39
    
Check a few things first. Starting with "do you have a free PCI-e slot in your computer". Next is "how expensive is a second 5570, and how fast is an equally expensive modern card? (Use sites such as this one to compare: tomshardware.com/reviews/radeon-hd-5570,2552-6.html ). Thirdly, check which games you play and which support crossfire. If your favourite games do not support it then you are going to be disappointed. –  Hennes Jan 13 '13 at 20:45
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@AlirezaNoori: Probably not. For one thing, you'll lose the ability to upgrade by adding a second card. For another thing, you'll be stuck with the same amount of graphics memory you currently have. And lastly, a new card will probably provide greater than double the performance of your current card -- a second identical card will provide less than double. –  David Schwartz Jan 13 '13 at 20:45
    
Thank you so much. I learned more than what I hoped for. –  Alireza Noori Jan 13 '13 at 22:18

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