TechReport found only
11W of power difference in L4D2 between the 470 and 5870.
All in all, while the 5850 is far more power efficient than the 470, the 5870's power consumption advantage does not offset the price difference/performance between the 2 cards.
Right now the only cards worth considering for gaming are GTX460 (and SLI) /5850 /470. It's hard to consider the 5870 unless the price falls at least $50-75. Also, with HD6000 series close to launching, I can't imagine spending $350+ on an 11-months old card.
Frankly, ever since 5850 hovered around $270 (even before Fermi came about), the 5870 was a bad deal at $375-399 imo. 5850 and 5870 also overclock similarly making 5870, once again, a crappy deal.