7950 = 600khash for 200 watts.  The 600 is on the low side too. 
5870 = 400khash for 200 watts.

7950's are way more energy efficient then a 5870 which will increase your profitability.

Check out dustcoin.com for quick price comparisons.  3 5870's producing 1.2MH/day will only making 1.44ltc/day.  Not sure what you pay for electricity but I pay 15cents and that would only be a $1.61/day profit. 

However if you got 3 7950's you would get 1.8MH/day and more then double your daily income and lower your ROI significantly.