there are a bunch of calcs
I use this one bitcoinx
a few important notes.
it doesnt take into account the fact that mining difficulty goes up every 14 days(it has normally been a day or two less than that actually) right now diff is looking to go up to 1580380 in about 4 days or so.. so you might want to use those numbers.(you can see the current guess as well as the number of blocks left until the next increase at bitcoincharts.com
you also will not have 100% uptime.
also this is a guess, even if you are in a pool, mining is based on luck and averages.. it is best to be in a pool with mh at your level.
you also have to guess what price will do, but it has been pretty stable for the past month or so.