well the GTX wouldn't be out until "end of march/begining of april"... so no luck there.. only the GX2 comes out in march 11th.
also...what is the order date shown on your receipt? is it earlier then Dec 13th? they will probably go by that date.
If you can sell it on ebay for more then 450 you could just pocket more money

I know people who sold their 8800GTS 320MB/640MB and bought a 8800GT and actually made money on the transaction. (and a faster card).
You could always keep on enjoying it.
You do know that evga has an "absolutely no cross shipping" policy for their step up do you? also quantities are limited by stock, so the 980GTX will probably not be an available step up option on release, only some time later.
In step up if your old card is valued higher then the new card (both go by their MSRP which is completely irrelevant to real prices) then you don't get paid the difference...