you can sell it and still get a good value for your card.
As far as currently increased performance, the reviews on anand and tomshardware suggest that the 280 is slower than a 9800gx2 and 8800 gt sli set up most of the time. Not to mention more expensive. On this front I don't think you are going to notice a great improvement or much of any until 2560 resolution.
See what happens with the 4870's reviews and then you will know if there is any reason to upgrade.