Regardless if it's being replaced as the best card by another, better card (8800 series), it's still going to cost a pretty penny to buy another because it still considred a top-of-the-line card.....
Reconsider about going SLi because, in many people's opinions, is over-rated....
Another thing to consider is that DirectX 10 is coming out pretty soon and DirectX 10 compatible cards along with it.... My suggestion is to wait and sell that card and buy a Directx 10 Card when they come out.....