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I'm looking to pick up a used computer. It is a Phenom 955 + 4 gigs of ram + Crossfire 2x Radeon HD 4770.

How would this Crossfire combo compare to a single video card?

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Note that the correct term would be "CrossFire" instead of "SLI" - SLI is applicable for nVidia cards only –  Sathya May 31 '10 at 6:13
    
the question is unclear. What do you mean by "combo" are you saying there are 2 Radeon HD 4770 cards installed? –  Evan Plaice Jun 15 '10 at 22:10
    
Yes, there are two installed. –  Citizen Jun 16 '10 at 17:37

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+100

Here's another article from Anandtech.com that compares the performance of one vs multiple GPUs.

TLDR:

In general, more than one GPU isn't that necessary for 1920x1200 with the highest quality settings, and the Radeon 4870 1GB or an overclocked GeForce GTX 260 core 216 are good options there. Slower single cards are fine for the high quality at 1680x1050 and multiGPU options are basically wasted at this resolution and lower except in Crysis. As we've seen in past tests, only those with 30" monitors will really benefit from multiple GPUs in their system for now.

Explanation:

You ask to compare it to a single GPU, which is a faulty comparison, since there are also GPUs that have two graphical chips on one PCB. It's one card, but has a much better performance (and off course costs a lot more) and is often recognizable by the X2 label.

However, I assume you want to know how much SLI/Crossfire will scale, regardless of the GPU model your using. Here the answer again is: it depends.

Not every game is made equal, so some games will have near perfect scaling with additional GPUs while others have marginal gains. The same goes for GPUs, not every architecture has the same scaling (though it's getting better nowadays), not every card has the same amount of RAM (or even the same type DDR3 vs DDR5) and new architectures generally perform better than older ones (if the specs are comparable).

I would suggest checking benchmarks to see what performance a said GPU has in the game of your preference and buy that one.

Here's an example for Crysis Warhead:

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Like CoD, Crysis favors NVIDIA hardware. The settings we're rocking require more than a single Radeon HD 4850 or GeForce 9800 GTX+ even at 1680x1050, so it's likely that many gamers will be running at lower settings than these. As with CoD, SLI sweeps this benchmark in terms of performance. The Radeon HD 4870 CrossFire pushes up against the GeForce GTX 260 SLI setup, but the core 216 or an overclocked GTX 260 setup would easily put some distance between them.

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In terms of scaling, SLI looks better at lower resolutions while CrossFire puts the heat on as resolution increases. Despite the fact that the 4850 scales at over 77% (which is very good), the higher baseline of the NVIDIA cards keeps this from making the impact that it could. At the same time, configurations with two 4850 cards perform on par with the GTX 285 and offer much better value.

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As you can see performance comes at a price, so it's up to you to decide what performance you want and picks the configuration with the best bang for the buck that gives that performance. If need be, you can always decide to upgrade by adding more. Note: make sure your PSU can handle more cards and that your case is large enough to fit more cards

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It depends on the single video card you are comparing the CrossFire combo against. In some scenarios, a single card could outperform a CrossFire/SLI setup. Here is a good post that explains in more detail:

http://www.tomshardware.com/forum/245454-33-crossfire-faqs

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