The 470 seems to be 5-10% faster than the 5850, so you have to decide if that performance bump is worth the extra money to you. Yea, the 470 uses much more power and produces more heat than the 5850, depending on your setup and preferences that may or may not be a big deal to you.
If you don't mind tinkering and overclocking than I think the 5850 is the card to get in my opinion. It seems that every reference 5850 will hit 900MHz+ with a voltage bump, many will get to 1GHz. The 470's seem to be a bit more finicky from what I've seen on forums/reviews. Some will overclock to 480+ performance, some will barely budge.
I really, really like my 5870 (the only reason I got it instead of a 5850 was because of a better than expected tax return) but if I were buying today I'd probably go with the 5850 and overclock as far as I can. It's the best bang for the buck of the performance/enthusiast cards in my opinion.