Cheers man!

Because you don't want all the extra power to go through your motherboard. It's the same reason anyone who uses multiple of any type of card (nVidia or AMD) uses powered risers.

Most motherboards are designed to handle maximum 2 additional cards boarded on them, which equates to additional 150w (+/-). More cards (75w+- per card) means more power is needed to power them - more current goes through the motherboard, which means increase in likelihood of frying said motherboard.

WU is normally about 90% of your KH/s, so by increasing your KH/s you also increase your WU, although different settings will yield different ratio between KH/s and WU - but don't expect it to greatly vary from the 90% mark.
Any configuration that you do with your miner should revolve around maximizing your WU and not your KH/s, so in a sense, your question is backwards and should be "How can I make my KH/s reflect my actual WU?".

1. Yes, but you should get powered risers if you plan to have lots of cards.
2. It won't reduce any KH/s
3. There's no issue to begin with (see #2)
4. They are plugged via a normal power cable to your PSU (usually a molex connector, although there are sata ones too).

"only 868 KH/s" - that's not so bad considering your power consumption should be (that was in my case) at around 250w per card, which gives it a 3.47 KH / watt ratio! Granted, you need to undervolt, but yeah, the potential is there.

I get 900KH/s now with 275w (3.27 KH / watt) per card with 1015/1250, 20481 TC 1.07v. Try it out.

FYI, your clock settings will vary to get optimal settings if you got Elpida or HynixMFR memory (I got HynixAFR).

No one mentioned them, and I guess it's because they are rather new to the scene, but https://www.kraken.com/ is also pretty good - been using them for a while and they got a reputable bank behind them.

I tried BTC-e but their fees are a deathblow for small-medium transactions (especially the SEPA one).

A couple of days later and I got 0.01796583 in my balance for 12h mining with 2.4MH/s. Very good!

I can't say for your card because I never owned a 5770, but you should go through this thread: http://www.overclock.net/t/1461916/tutorial-ultimate-guide-to-tweaking-amd-cgminer-3-7-2-kalroth-edition-sgminer-instructions-included-aswell-get-the-most-hash-out-of-your-gpu

It's an excellent guide though you might need to apply a bit of changes where it's needed so it'll fit your own card settings.

Well, I also got R9 290 with HynixAFR, so it might have account for the difference in speeds you and me get (in case you don't have HynixAFR youself).

I suggest for you to set back the voltage to its original value and than play with the core and mem speeds, once find KH/s you like, start undervolting. You could always go the reverse route if you wish - set a certain voltage (never set it above stock!) and than find the optimal core/mem clock speeds that give you the most KH/s.

You can start by trying mine: 1015/1250 with 1.070v - those give me 900KH/s with 275w usage.

But I wanna emphasize again that if you got different memory type (Elpida or HynixMFR) than my clock speeds would probably net you different results, most likely worse so you'll need to do some trial and error until you find the sweet spot.

It's not that weird. If memory serves me right, than at stock values a single R9 290 eats about 330w-350w, 2 of those will make it 660w-700w and when you add cpu, mobo and ram on top of it you exceed the limits of your PSU. A quality PSU can provide more power than it lists as, but it's not meant to exceed its limit for long periods of time (weeks in your case).

I can't say for sure if your PSU is toast, but try undervolting your R9 290 with stock clock speeds and see what you get.

A really conservative settings I used to have were 911/1250 with voltage down to 1.023v, those gave me like 225w per card but hashed at like 800KH/s.

Did you undervolt? a 750w can be enough for 2x R9 290 (and of course mobo+ram+cpu) if you set it correctly and go a bit conservative with the settings.

Anyway, post your settings here.

You need to compare it to your WU not KH/s...

Usually a display driver crash is due to misconfiguration of the clocks speeds and/or misconfiguration of the voltage and/or bad drivers.

So post your config file here and and driver version you're using.

I got R9 290 and a total of 4GB RAM in my system and everything works fine.

Let them think it matters. Ignorance is bliss.

DONT overvolt... stock is 1.1v so that's the highest value you should ever set.
Remove 'shaders' as it has no affect when you set thread-concurrency.

Remove that gpu-vddc from the config and set your core to 800 and memclock to 1250.

If your PC still crashes than download and install MSI Afterburner (ver 3.xx - beta) and let it detect your clock speeds and voltage (you might need to restart after installing).

Stock voltage for the HD 6950 is 1.1v, so if MSI Afterburner reads anything else you might have a problem there (I assume you didn't flash any bios) and you would probably need to set it manually back to 1.1v and than change the clock speeds back to their stock settings (if you haven't already - 800/1250) and work from there.

Not here to badmouth hashcow but I really dunno how people get 0.015 BTC here, I tried it for 12 hours and I got a mere 0.00027175 (confirmed + current) BTC for 2.6MH/s...

Sorry, but why are you saying those coins are undervalued? Severely even. What's their current market-size? And I mean their real market-size - which retailers actually accept any of those as payment?

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