to answer the initial question: yes, it is.
let's say you only get 2x5850 (not 5870s) for $800,
2 5850s get around 700MHash/s and the rig will need ~375W (probably less, but let's just be sure),
at (very expensive) $0.3 per kW/h you'll pay $2.7 per day
at current difficulty the expected average output is 2.88BTC per day,
at current market rates (of roughly $6.5) that's $18.72 per day.
$18.72 - $2.7 == $16.02 per day
16 / 800 == 2% ROI per day
show me an investment opportunity that pays you 2% per day and i'll join it.
Actually, I did calculations too and:
Today Im earning 23$ per day (after paying bills) then +35% difficulty
In 10 days - 16,1 $ then +30% difficulty
In 20 days - 11,5 $ then +25% difficulty
In 30 days - 8,3 $
After 40 days ~600$
After 60 days ~713$ (few more difficulty rises)
I just assume that price wont change and Im good in predicting difficulty rise.
To get 23$ per day with current difficulty you need to have around 3.8 BTC per day, which gives 920Mhash/s (3x 5850)
If the price stays at 6.8$ most prolly u wont get back your cash, so your investment is risky.
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) and that my wallet holds a meer 6.73 BTC. 
