Originally Posted by antuk15
I'd of just bought another cheap 5770 for x-fire and then bought a 7000 series GPU when they release...
That's a notable, affordable option. As for 2x 5970's -- your CPU might bottle neck it a tad. And, such a setup would essentially be quad-fire because of the dual GPU/board design.
The 5xxx series had pretty good scaling in dual-card layout, ok in triple, and downright terrible in quad. However, because everything is crammed into two cards it seems to do quite a bit better than four stand alone cards as referenced by this Guru3D review of Crossfired 5970s shows
For sheer power, Crossfired 5970s would be quite a sight to behold and would easily last you another 2-3 years running at 1080p and most eye candy at mid to high levels. But it all comes back to your CPU. Four GPU dies is a lot for that aging proc. And if you're not wanting to upgrade that right now, you're earlier conclusion above -- going with a second 5770 -- may be your best bet.
Of course, you could always grab a single 6970 (or 6950 and flash the 6970 bios for free power) if you want to stay on AMD's side, or pick up a GTX 570 if you venture to the green camp. But again, the Q6600 comes back to bite you. Asking for another 3-4 years of GPU upgrades with the same CPU is really pushing the realm of reality.