I bought a 5770 7 weeks ago that paid for itself in 6weeks(not counting electricity), I just sold it because I'm moving xcountry so ended up with a 100% return on investment.  Your numbers are good I believe, and 5xxx cards are a good choice so go for it. 
disclaimer: I started mining just as the price started spiking and my pool had a long streak of good luck so don't expect as large of a return as I had.