With bitcoins at about $0.20, a block of 50 is worth $10, so after 40 days the card would be paid for. That may be a little optimistic (especially if everyone gets 5850s for Christmas) but at least it would be fun to be generating blocks.
don't forget, that within those 40days the difficulty will rise, so if the price of bitcoins stays at ~$0.2, it might take slightly longer for the card to pay off.
besides that, the 5850 isn't a bad deal, ~230Mhash/s at default clock, ~300Mhash/s overclocked.
if i had some less expensive energy, i'd just buy some more.