Yeah, I like coastie here too. I know it gets very dramatic when we see these big games on the horizon, and can be enough to put you into a fit of anxiety...but just breath a bit. The best answer to any of these questions like this over the decades advising folks: Wait for the release. Try it. And if you feel the need for the upgrade at that time, then do so at that time. To adjust hardware now for a release due later just isn't logical.
Just think, when that game is out you'll be able to run it on the 5870 you recently got in the first place, and perhaps have cheaper 6xxx cards to pick from than you do now because the 7xxx might be weeks away from hitting the stores. All in all, you absolutely KNOW it'll play on your 5870. How well? Well, does that
really matter? On a small scale, yes...but on a large scale it's a big
NO. It's going to be a beautiful game, latest and greatest hardware or not. I'm eagerly awaiting it's release and I'm going to play it with some GTX 570's and know I'll be fine.
Wouldn't it just suck if it's really really pretty, but the game play sucks badly, and you upgraded for
this one game? Dude, this harkens back to when SLI made a come back. The masses just needed that 150FPS for that DX9 glory because 98 just wouldn't cut it. Or more correctly, Doom 3 in '04. Many upgraded for that massive game, just to be running around in the dark the whole time. Huge sucking sound from people forking out dough for hardware due to that title.
Not what you want to hear, but I feel it's good advice. At least sane and logical

We can talk for pages about what the game will utilize, what hardware will allow these utilization, so on and so forth...and end up with one summary in the end; or one nagging question ("I would have been fine with my old parts, right?")