Wait for reviews.
Its faster than a 5870, but by how much depends on the game, so wait for those reviews, don't worry were not gonna run out.
I am irrationally wondering about the same thing (irrationally because I had told myself I was going to wait for fully new-generation, 28nm cards). The suggestion of a 2nd 5870 doesn't make as much sense to me - crossfire scaling in the 5800s never worked as well as it should have (or at least, it didn't the last time I looked) and power consumption and heat would be a lot worse.