Best GPU for mhash per electricity usage?

[Kennji]

6930 gets speed like that for that cheap?!  Interesting.

I still think I am going to roll with 7970s for now.  7990 is too much ... but 400 I can roll with because they will hold their value for a long time like a nice used car!

I see 5850s out there for 180 range still.  That is INSANE!

[Kennji]

At todays prices ... you will make about 7.50 per day with a 7970.

53 days range to pay it off, if the market stays at 105 USD per BTC.

That is not too bad in my opinion.  That is including power cost on this calculator I found.

[Lauda]

At todays prices ... you will make about 7.50 per day with a 7970.

53 days range to pay it off, if the market stays at 105 USD per BTC.

That is not too bad in my opinion.  That is including power cost on this calculator I found.

Wrong because of 2 reasons: 1) high chances the BTC per USD rate will drop.
more importantly 2) you didn't calculate in increasing difficulty factor.
Use: http://dev.bitcoinx.com/profit/
even that's not accurate, since in a week or two you already have a diffciulty of 11+ million

[Kennji]

What makes difficulty go up so much??

[AlphaWolf]

What makes difficulty go up so much??

People with ASCI miners, and increased awareness of Bitcoin resulting in more people mining.  The more hashrate you throw at the network, the more the difficulty increases to compensate.

[Signus]

If you're investing in GPU mining, you have to go big or go home at this point. Remember to check out the hardware comparison guide, and see where you get the most MH/$.

Either a 7950 or 7970 would work very well, as long as you buy bulk.

[Cryptorig]

Hd7950s! 7970 use a lot more power, and cost 30% more for 17% improvement in H/s. hd7950s only use around 200w each. Much better long term. I use 7950 in all the rigs I build, unless I have a customer that can't be talked out of using 7970s

[rovchris]

It depends on how much you can overclock the card while decreasing the voltage to it.

The entire thing is a balancing act.

The more powerful cards you have  - the fewer motherboards you require.

There is a big overhead in running more machines with less powerful cards in it.

The best you can do at the moment is 4 * 7970 in one machine - that will give you the most mh/s per watt

This will pull around 1150 watts at the wall and 2.8gh/s

There is a bios memory limitation in installing more than 8 gpus in one machine. a 7970 counts as 2 gpus.

If you try and connect more you the card will show up in device manager with an error against it.