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.
you are missing the time you put into setting up and watching the rig. that is gonna be a couple of long days work depending on your experience. if you would work for money and buy bitcoins you would be much better off. so the question is: how much fun is it for you to play with the rig?
Two long days to set up a rig? Good lord, I don't think it's ever taken me that long. My first build took all of 3 hours to put together, and the most recent ones have been done in under an hour. Install OS, install drivers, ready to go in under 2.
Anyone who takes two long days to put together a computer really shouldn't be putting them together to start with.