Well, thinking reasonably I don't think the 5870 is worth the extra money over the 5850, but of course I threw that out the window when I got my own 5870
. Anyway, yes the 5870 should last you a little longer compared to a 5850, but it's kind of like comparing the longevity of a GTX 275 vs a GTX 285. Anyway like flyinfinni said, a 5870 makes more sense for you simply because it is a more significant upgrade for you. While a 5850 is still a decent upgrade, I'm don't know that it's justified in your case.
So yeah, if you're gonna spend big bucks then may as well go all the way with a 5870