King, the 5870 is not $500...
All of the 5970s are reference boards, so it doesn't matter which one you buy. AMD released a GPU Tool which you can use to increase voltage.
If you want to wait, a Sapphire TOXIC 4GB version should come out, but if you want one now, XFX is not a bad choice.
As for crossfiring options, 2xHD5970 won't scale well. You should probably just do an HD5970 + HD5870 TriFire.