HD7970 manages something between 650~750MHs (my two cards peak at 680~690). Let's take 680 as the base line.
Following the calculator here
http://dustcoin.com/mining, it will make you 0.0339 BTC per day ($4 at today's rate) disregarding power costs. Expect about the same income in LTC as 7970 (at least for me does not go above 540KHs)
Just remember that difficulty increases about twice a month and that the next increase will shave about 20% off these numbers.