Buy yourself a radeon hd 5870 card for about $200. With 350mhash/s you'll get 1.3 BTC/24 hours. That's 39 BTC/month. It's about $273 with current rate of 7 USD/BTC. Not counting electricity bills. Most likely the BTC exchange rate will go up after difficulty increase, so you'll get even more than $73 profit in return.