For the 6990 @ $722, based on current rates ($14.5 @ Mt Gox) for a 6 month time period:

Revenue per day: 7.29 USD
       ... less power costs: 5.76 USD
 
Revenue per time frame: 1330.60 USD
       ... less power costs: 1051.18 USD
 
Hardware break even: 125 days, 10 hours
 Net profit first time frame: 329.18 USD