I try to help you guys out as good and as short as I can:
The Sapphire 5850 I talked about supports Eyefinity and supports 3 Screens (1DVI,1HDMI,1*DP) - For what I read there is one HDMI>DVI adapter included, if you want to connect 3 screens, one has to have display port support.
Price Comparison for Sapphire Radeon HD 5850 Extreme
For high resolution texture work you might want to grab a card with 2GB vram, which unfortunately is quite more expensive.
The Nvidia padadox:
The fastest sane 200 series card is the 285, which costs 200 Euro while the GTX560TI costs 190 Euro and is almost twice as fast - in DX. In OpenGL the 285 is ~4x faster.
So, unless you need CUDA/3DVision/Tesselation, Nvidia currently is NO option for Blender.
Only if you got the big budget, then you can buy for instance a Quadro2000 (~400 Euro) it would be fast in OpenGL and capable of CUDA. Seems too convinient for Nvidias Quadro sales to be a coincidence?
(Sidenote: Tesselation is also good with the Radeon HD69XX based on the Cayman chip)
Radeon graphical glitches:
Opposed to the general believe, the problem is neither with Linux, nor with Blender, the problem is the Radeon drivers. Since the HD5800 series and the new drivers that came with the Cypress chip the issues should be gone for the most part. The AF still is flickering compared to Nvidia, while the Radeon´s AA is far superior to Nvidia.
For the Radeon cards and my conclusion about speed:
Unfortunately there are virtually no pure OpenGL benchmarks for cards, but usually (unless nvidia cripples the cards scnr) the OpenGL performance is not far off the DX performance. I guess you´ll have to wait until IDsoftware releases RAGE to get OpenGL benchmarks again. Also consulting CG magazines doesn´t help, because there they either test FireGL or Quadro(FX).However:
the 5850/5870 are based on the Cypress XT chip.
the 6850/6870 are based on the Barts XT chip.
The Barts XT chip basically is a Cypress chip on a smaller die, with a bit better performance:watt ratio as sales argument to increase AMD´s profit. There have been minor modifications and improvements in the architecture but thats about it. It´s a “refresh”
So, benchmarked over several games and applications you get a performance-rating:
GTX580 - 400 Euro - 100%
HD5870/2G - 220 Euro - 77%
HD5870/1G - 170 Euro - 77%
HD6870/1G - 160 Euro - 71%
HD5850/1G - 145 Euro - 66%
HD6850/1G - 130 Euro - 61%
And the Sapphire card:
HD5850/1G - 110 Euro - 77%
From the 6850 to the 5870 we speak of ~10-20fps difference in games with ~80-90fps or ~5-7fps difference in games with ~30fps.
Not hard to see that the 6800 series is unattractive and that the Sapphire Extreme has the best performance/price ratio.
Furthermore you should be able to OC the Sapphire 5850 to 5870 performance quite easy. I can´t say it for sure, I haven´t got mine yet, but it´s said that the card has a free voltage controller, so OC, or UC for the energysavers should be a cakewalk. (note that most 5800 cards have locked voltage controllers in the second revision or with non reference cooling solutions) And don´t be afraid to OC a card nowadays, it´s no rocket science nor bad for the card - it´s rather good practice.
The Radeon HD6900 series - the ugly duckling:
This card seems like an emergency card to compete with nvidia.
In DX the Nvidia 500 series is far superior to all AMD cards so they had to come up with something to compete quickly. They simply made the Caymen XT chip without many thoughts. The cards run with 930+ MHz (1GHz is the magic number for GPUs) on its limits. The chip is huge, so they had to kick up the voltage to a degree where it is on it´s limits too. Both those factors make the cards: loud, hot and barely OC able. They are at the limit and basically now AMD has cards that are exactly what they made fun of when Nvidia introduced the GeForce400 cards. On top of that the fastest AMD (6970) costs 275 Euro and is not as good as the GeForce GTX570 (which is OC-able, cooler and more silent) and costs 270 Euro.
AMD´s last two generations (6800, 6900) aren´t attractive products and Nvidia produced OpenGL crippled cards.
An impossible situation.
For Budget:for Blender, OpenCL: Sapphire Radeon HD 5850 Extreme
for Blender, OpenCL, Gaming: Radeon HD 5870
If you got money:for Blender, OpenCL, CUDA: Quadro 2000
What I am doing:
Like I said, I ordered a Sapphire 5850 and I am confident that it works together with a GeForce in a system as long as screens are connected. Latest Linux distributions and Windows 7 support to have more than one display driver installed. I had a talk over at the Octane forum and at least it´s possible to run OpenCL on a mixed system. I read that also CUDA is no Problem. If you don´t have screens, dummy DVI connecters do the trick.
So I´ll use the GTX470 in my system for CUDA and gaming, the HD5850 for Blender and the 5850+GTX470 for OpenCL. Seems overkill, but really, both cards together are 250-300 Euro, used even cheaper, and my GTX470 runs stable with 800MHz(Reference 600MHz) without touching the voltage. That takes it somewhere between GTX570 and GTX580, close to the GTX580 for ~150-170 Euro. You need an aftermarket cooler for that though.
If I don´t get the cards to run within one system properly, I´ll get an AMDx6 with mainboard and 8GB DDR3 and put it in one of my spare systems with the Radeon as Blender/OpenCL/CPU raytrace system, and keep the Intel/GeForce system for CUDA and gaming.
It is still cheaper than buying a 400 euro quadro.
As soon as I got my card and tried to run them under Linux and Windows I´ll report my findings.