2x6950 or 1x7950?

[Elmojo]

Hi All,

I'm just getting into BTC mining, and need to upgrade my video card.
My current PC is a Core i7 930 @ 4.0Ghz, 12GB RAM, SSD, and a GTX480 (factory water cooled).
My PSU is a Corsair HX1000, so no worries there.
I was originally planning to upgrade to a 7950 with a waterblock, which will run me just under $600 and from what I've read should net about 600Mh/s when overclocked and tweaked.
Today, I saw that sunbreak is selling a bunch of new 6950s for ~$220 each plus shipping. He claims they will run at 360-400Mh/s each, for a net of roughly 700-800.
Later today, I ran across a post reminding me that a 5970 will rock about 750 Mh/s. I can get one used for ~$450, and add a water block for about $120 if I choose.

I'm so confused! lol

Can anyone offer advice? Am I better off mining with 1x7950, 2x6950, or 1x5970?
My power costs are roughly $0.12/Kwh, and my machine stays on 24/7 already.
I have 2 PCIe slots (1 currently open), so I could leave my GTX480 in there and install the 'mining' card, if that's an option.

Any advice appreciated!
-Elmojo

[Lazer]

First things first, u dont need an overclocked cpu for mining. in fact you may wanna underclock while mining it to save some power.

go with the 6950s they have a better performance to energy consumption ratio

[Elmojo]

True, but this is my primary rig, used for Editing, CAD, gaming, etc...
It already runs 24/7, so the only extra power use will be from the GPU.
Do you have any data that shows those cards being more efficient?

[Lazer]

Do you have any data that shows those cards being more efficient?

https://en.bitcoin.it/wiki/Mining_hardware_comparison

look at Mhash/J per card, the higher the number the better

[Elmojo]

I've stared at that page several times, and still can't seem to make heads or tails of it!
There seems to be several metrics for what makes a "good" mining card.
Based on your criteria, the 6950 is actually the worst Mh/J of the three, or am I missing something?

Model  Avg. Mhash/s    Mhash/J    Mhash/s/$
5970        655.83              2.01       0.90
6950        360.62          1.8               1.44
7970        625              2.89       1.13  <-Overclocked 7950 should be within 15-20% of this

If I'm interpreting the chart correctly, the 7970 is the clear winner on efficiency, while the 6950 is the best hash/$.
Taking into account the lower cost of the 7950 compared to the 7970, it looks like it (7950) is the best choice.
Of course, that chart lists the 5970 as a ~650 hasher, while I've seen a couple references to them being more in the 750 range.
That could make a difference too.

Somebody please set me straight here!