keep the hd6990, it should definitely go on par with most of the gtx680s and hd7970s and even beat them in some instances. The difference in performance wont be noticeable at all unless you decide to get crossfire or sli of hd7970/gtx680.
IMHO, you should wait for what nvidia gtx700 and hd8000 series has to offer its only a few months till they hit the market. Selling off your hd6990 for a newer gpu would be waste of money.
do you think i can add another 6970 ,as i only have a 750w psu