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Hello,

I have HD 6850 atm, 2x HD 5850 should come to me in 2-3 days,

I wonder what drivers and which ati stream version should I intall?

Do I need the same drivers for each gpu?

In which order should I install/uninstall cards and drivers?

Should HD 5850s be crossfired?             
Would you recommend exchanging my HD 6850 for a HD 5850 which is a bit faster, so all 3 gpu's could work in crossfire (my motherboard would support it).

What temps should I expect?
1 or 2 cards can be connected with a 15 cm PCI-E ribbon/extender, so I guess this should lower the temps somewhat. My case is some chieftec, somewhere between midi and big tower, but there isn't much space inside.

And last question, least important to me atm, because my PSU is only 550 watt Corsair (~85% efficiency) so I would have to underclock my overclocked Q9300 (3.25 GHz now, 2.5 GHz was stock):

How do I overclock the cards if they're from 2 different manufacturers? HD 6850 is a Vertex (Powercolor, as far as I know), HD 5850s are Sapphire.

Can I overclock each card separately?

Awaiting your anwers,
BTC mining noob,
Malevolent

This is what I did:

Get the drivers 6850 and get it all up and running for that. You dont need 5850 drivers.

One set of drivers is all that is needed

Order doesnt matter, it will auto update (fyi you need dummy plugs or monitors for guiminer to initilize the card to be visable for mining)

Dont crossfire it actually makes it mine slower.

If 5850 is faster, ya do it - dont crossfire.

Temps OC'd probly in the low to mid 80s

MSI Afterburner will do multiple cards, go to settings then pick the card push ok, adjust it, back to settings and pick next card to adjust.

I personally think 3 cards on a 550w cosair is to much, it cant handle that but I could be wrong.

I agree with shata, there is a good chance you'll run into power supply problems.

I've been a big fan of PC Power and Cooling ever since I read a MaximumPC (long ago) that stated they have never had one fail in the lab, except the one they dropped. Newegg even has some now (http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=50009830&IsNodeId=1&Manufactory=9830&bop=And&SpeTabStoreType=0&Order=PRICE&PageSize=20) that are not over the top expensive.

Also, if you are running Windows you might need a couple of these (http://www.overclock.net/folding-home-guides-tutorials/384733-30-second-dummy-plug.html). Cheap and easy to make, no soldering required.

Thanks for answers, yes I knew about dummy plugs and PSU.  :]
the 3 GPUs @ 100% load with underclocked VRAM, CPU @ stock, 2 HDDs and 1 DVD, Asus P5Q-E and 4 sticks of 1 GB RAM each, should take together ca. 500 watts of energy, so my PSU will be working under pressure, so I am planning to change it. I live in Poland, so the prices are a bit different here, I'm looking for some second-hand Corsair or Be quiet! (750-850 W). I can buy a new Corsair TX 850 for about 100€ or a new 800W Corsair CMPSU for about 85-90€, but that's still expensive.
I also thought about OCZ Gamexstream 850W SLI-Ready for 65€ but I heard they make worse PSUs than Corsair..

Does it matter what kind of pci-e ribbon/extender I use?
I heard that powerful GPUs need some quality extenders which are 2x more expensive.
Is it B.S. to spend more money or is it true?

Corsair TX 850 can easily handle 3 graphics cards.
I got it powering my Core i7 860, 2x 6950 unlocked with high overclock of 800@960MHz and 5830 @970MHz (uses more power than 6950's).

+1 Yes... 100% agree, I use nothing but pc power and cooling actually. Most reliable psu you'll ever own and should it break... ohhh ya thats right 7yr warranty.

Rig 1
Pc power n cooling 850w
Visiontek 5870
XFX 6970

Rig 2
Pc power and cooling 500W silencer
Diamond 5770

I have not done ribbon setup but quality should not really matter. Just get normal one everyone else gets.

Okay, thanks, one (hopefully) last set of questions:

How is using my PC going to look if I want to play games or watch movies? 

Which GPUs are going to be working?         

Would it be possible to 'turn off' HD 5850s and leave only HD 6850 to work if I want play only games, watch YT, movies, etc. (non-mining stuff)? I cannot afford to have two PCs and I can't imagine games to be working properly with 2 non-crossfired HD 5850 and HD 6850 (gpu from next generation) :-)

Which drivers and which ati stream version will work best on my setup?

If you end up using multiple monitors just make sure 6850 is connected to ur PRIMARY monitor and it says desktop 1 in the driver. Every game your gona play is gona default full screen to desktop 1. Also you cant even crossfire a 5850 and 6850, only same generation 6's with 6's and 5's with 5's etc.

Honestly im not that bitcoin crazy about the whole driver version thing, I just have newest ati drivers from their site. (but their may be small mhash/s increases between drivers but im just to lazy to go through all that work)

No. I'll be using only one monitor and dummy plugs . . .         

I don't do it often, but I've had no problem gaming on one card and mining on another.

My only experience with a pci-e extender didn't go well. I don't know anything about it because someone else purchased it, but if I had to make a guess it was the cheapest one on the planet. Couldn't get it to work in any slot/card combination. I'd guess there are a lot of things that come into play on their success rate. The motherboard was a decent MSI board and the cards were Sapphire 5830's. If I was going to get an extension for myself I'd error on the side of more expensive (within limits, no need to get ridiculous).

The OP may want to consider using a second, less expensive PSU in addtion to the 550W he already has. Some people have done so succesfully according to posts here.

Yes, this would also be a lot cheaper for me, so I considered this but I am afraid of the doubled risk.

Another PSU multiplies the chance of frying some (or all) components by 2. Even assuming everything is correctly connected and grounded.

Back in the day I lost an Athlon 1GHz Slot A CPU (yeah I know I'm old, but it was the first chip to break into GHz) to a dual power supply experiment. Everything worked fine, my cat stepped on the power strip running the fans and drives. It was a sad day and a hard lesson. I learned something important. Hopefully you can learn from my mistake too.