they game less well and dont sell very well, and at lower prices.
I've seen benchmarks that say they are very close. So close you won't notice a difference while gaming.
I've also seen completed ebay auctions with used 5850s selling for more than you could recently pay for a new xtreme at newegg, so resale seems fine to me.
prices in ebay can be veryy high, doesnt mean they sell at that price. I mean to compare them crossfire wise, the 6850 scales much better