^ That all depends how good your gpu(s) are. That will determine how fast you can mine for bitcoins. Figure out how much power your computer will use if its on 24hrs and see what your hash rate is. Then work out if its worth it. Ie, a single 5850 could net you around 300 Mh/s. And at the current difficulty rate of 1379223.42967, it will take 5 days of 24hr mining to make 1 bit coin. That one bitcoin is currently worth $17 or so. To achieve 1 bitcoin per day you would need a hash rate of at least 1500 Mh/s.
http://www.weusecoins.com/