Another vote for one 6870 now then one later on.
That's exactly what I'm doing.
I also recommend the card jjb8675309
linked you to.
It's a wonderful card. Looks great. Performs amazingly well.
You also get an awesome deal on price with that MIR (which I just remembered, I need to send mine in).
XFX has fantastic support as well. You also get Shogun 2, free.