Another good calculator:
http://www.bitcoinx.com/profit/It auto-fills current difficulty; just put in your info and read the results.
Also keep in mind that the difficulty has recently been rapidly increasing (due to ASIC miners coming online) so the amount of BTC you generate will probably continue to significantly decrease as time goes on. Throw an unstable USD/BTC exchange rate into the mix and mining starts looking like a complete gamble.
But there isn't much risk if you already own the hardware since the hardware wear and tear is minor if you keep fan speeds around 85%, run your rig in a cool environment, and make sure you set your miner to prevent the card(s) from overheating. Simply stop mining if your electricity cost starts exceeding your mining returns and your loss will be minimal if at all.