It would depend on how much yhou can purchase one for, how much it costs to run compared to how much it can mine.
There is a comparison list here (also has config/command lines for cgminer running scrypt etc)
https://github.com/litecoin-project/litecoin/wiki/Mining-hardware-comparisonthe 7990x2 is two 7990 strapped side by side.
over here I'm about to pick up a 7870 for $200AUD
If I were to get a 7990 that would set me back $1350AUD
So basically I should expect 6.75 times the performance at that price.
From the above link, it seems that the 7870 does around 400m#/s, whilst the 7990x2 does about 2200 to 2300. so a 7990x1 should dig up around 1100m#/s. I would say stock would be around 900m#/s.
That is 2.75x the rate of a 7870
Then comes power effiency
Power in australia costs $0.27AUD kWh, ($0.30USD kWh).
I think in the USA its around $0.17USD kWhb
I mine BTC, which consumes less power to mine per hash rate than LTC.
My 7870 pulls 404mhs at around 110kWh to 120kWh at 80% efficiency (bronze corsair power supply running at around 25% load.
For LTC this will be higher - I'd estimate around 140kWh as its TPD is around 160kWh (so reduce that down). LTC mining requires ram, whilst in BTC we can downclock the ram as its not really important (BTC/SHA256 relies upon integer arrithmatic, but not much data transfer, LTC/Scrypt relies upon data throughput aswell, so you will generally want fast ram)
So 7870 @ 400m#/s @140kWh for $200
the 7990 is rated by AMD to require a 750kWh power suppy however youd probably kill that pretty fast as its TPD is 555watts, and if your motherboard sits at idle and even if you have a 35W chip, you are still at 590/750 so you would have a pretty inefficient power consumption. If you were mining bitcoins with downclocked ram you might pull the card down to 300watts, but for litecoiin my guess is you would be pulling 350watts maybe - especially when you overclock it to reach 1100m#/s, possibly 1200m#/s at 400 watts on a 750kWh powersupply.
So 7990 @ 1200#/s @ 400watts for $1350
Comparing the two, in Australia you would be crazy to purchase a 7990 unless you only had one PCIe slot.
Even then you'd be better off buying a new motherboard - or taking a holiday.
I think from what I've read, a 7990 is around $900usd overseas? and 7880s are $250 or something.
In that case, it would still not make sense as you are still only getting 3x the performance, at more than 3x the cost, and more than 3 times the power consumption.
You cuold be better getting 7970s.
Again, in Australia for bitcoining, it doesn't work out either due to the increased power and cost (they start at around $380) but only produce about 1.5 to 1.7 times the m#/s. However they use about 1.5x the power so m#h/s they come out equal to slightly ahead. We still have to make up an extra $60 of profit for that card to make it worth while though.
However there is also the resell value. 7970's and 7990s should hold their value longer than 7870s. I'm still getting 7870s as at the end I'd probably use them for gaming anyways.
Please not I started typing this before there were anyreplies - I just got the:
Warning - while you were typing 5 new replies have been posted. You may wish to review your post. so I haven't taken into account any of the previous replies.
