We've been dealing with CrossfireX reviews ever since the beginning and surely a lot has changed over the years. The one recommendation we always gave you guys is to keep it simple at 2 GPUs maximum, as after 2 GPUs in a CrossfireX setup you quickly run into weird anomalies that can be irritating. AMD limited the 6800 towards 2-way Multi-GPU gaming as such was a very peasant experience. Most if not all games scaled just really well and the games that are a little GPU bound can scale at 1.9x ... which is very impressive to see.
So over the years Multi-GPU support has improved quite a bit, AMD still isn't up-to snuff at the level of NVIDIA though, multi-GPU supports still literally and directly remains the Achilles heel of ATI's Catalyst drivers. For years now we have been requesting user-based multi-GPU profiles. See when a new game comes out, multi-GPU support will not be supported by the Catalyst driver. ATI will always need to update this through drivers or downloadable profiles. What ATI needs to do is to allow the end user to make custom profiles per game. A small tip, renaming your game-executable towards a game title that is supported can, not always but often, help out.
Now we also do have warn you about the fact that multiple GPUs make more noise and heat inside your PC, so make sure you have some good ventilation inside your PC. Noise wise the R6870 was by far the best, you can expect plain silly old normal noise levels. The R6850 setup was more noisy but remained acceptable and not irritating, but it was borderline though.
Performance wise we see really good numbers, with two 6870 cards you'll see 27.800 points in 3DMark Vantage ( GPU P-score) and 23K5 points for the 6850 setup with two cards. That's sincerely high performance for cards in this price-range anno 2010. Relating that to games then, Dirt in DX11 scored 112 FPS at 19x12 8xaa/16xAF on the R6850 and 124 FPS for the R6870. Considering 60 FPS is all you need you would be future proof for a while alright.
Battlefield Bad Company 2 scores 89 FPS (6870) at 1920x1200 and 8xAA, that's just brilliant as that is a HUGE framerate with 1.9x performance increases. Fact is, you'll beat any single-GPU solution to date in terms of performance, and where the title really is multi-GPU savvy it'll kick in even harder.
Power consumption wise we don't see anything shocking either really, two 6870 cards peaked towards 275 Watt (graphics cards only) which is slightly above one GeForce GTX 480, these are all 'okay' values considering what we are doing.
So let's round things up, personally we always say stick to one or maybe two high-end cards as there is so much less driver fuzz to worry about. It's like this with ATI, once you pass 2 GPUs you'll often find yourself compromising a lot with new game titles versus multi-GPU support. And this is why today's tested products in 2-way CrossfireX mode work so well.
AMD really does needs to step it up a little more on better and even faster game profile support AND user based and created profiles, period. But sure, we also have to acknowledge here, ATI these days has very solid driver releases and release hotfixes and Crossfire support profiles to address to this concern. Credit where credit is due, the overall experience definitely is very interesting.
If you can pick up a second R6850 or R6870 for the right price and can life with some small catalyst driver incompatibilities (as in the future there's bound to be some), then sure .. we'd give a setup like shown today our two thumbs up. Its a relative easy and affordable way to double up on your game performance, if it's properly supported of course.
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