There are several mining profitability calculators available. In order to obtain useful data from these, you'll need to predict how the price of bitcoins and the mining difficulty factor will change. Price charts are available at bitcoincharts.com, and difficulty charts are available at bitcoin.sipa.be. There are also charts of the price to difficulty ratio available at bitcoinx.com.

I`ve made a simple calculator that can calculate estimated mining profitability (given that you know how much power you are using and how much it costs you). http://tpbitcalc.appspot.com/ But the problem with all mining calculators is that the Bitcoin economy can shift very often. Difficulty and bitcoin exchange ratio change quite often, and the latter sometimes very rapidly, so projecting too far into the future has to be taken with a grain of salt.