keep checking on a regular basis and you'll find brand new 5830 cards listed from $130 to $160, which is very legitimate for this card
Anything close to $150 is not legitimate. 6870's go for about 150 and they get the same megahash and require less electricity.
Very close, but 5830s do cap out a bit higher than 6870s. Mine peaks above 340Mh/s, for example, and I know many owners get even more out of theirs. While it's not by much, the 5830s have a little bit of a performance edge over the 6870.
Then again, as you said the 6870 drawing less power may offset the difference in performance.
However, I'm having trouble finding a 6870 card at $150. The ones I've come up with on the usual places seem to bottom out at $170. Newegg has some that go for $155 if you're willing to wait for the mail-in rebate, but at that price the 5830 seems the better choice with no rebates to wait for.