I won't be as unfriendly as some user. With those nice GPUs, you can except around 30 DCR a day with the current hashrate and difficulty. That's around 0,15BTC per day, so quite nice ! If you feel generous, don't hesitate to PM me !
30?... never
I use 2 6990 dual gpu's and one 6970 and I get about 4-4,5 DCR / day.
280X isn't 7 times as fast.
You can say 900 Mh/s = 1 DCR / day
A 280X produces around 2,2 GH/s. I just picked the numbers from Suprnova.cc, and generally they're quite exact. However, it can fluctuate throught the time, that's why I put a disclosure.
What miner are you using to get that from a 280X?
2,2 GH/s would be ROI in nearly 1 month for the card.