I'd buy one 8800GTX now, as games coming out before the new cards wont push it much. (If you can, buy an OEM, as they cost like £60 less, yet perform the same. They just don't have a box)
Then, when the new card does come out and its the right price for you, sell the 8800GTX for a couple of hundred, and buy yourself the latest. Again, you shouldn't need SLI for this.
So, it should end up cheaper in the end.