I bought my 6990 to mine with and for a better cards for the occasional gaming I do. I consider this a sunk cost from wanting a better GPU and it's in a PC I already own, so I don't consider that an expense. In just over a month of mining, I earned enough to purchase additional cas fans, a riser cable and a 5830 to use as a second card.

Personally, I anticipate mining bitcoins and slowly addding additional hardware from profits and seeing where that goes.