http://www.techpowerup.com/reviews/HIS/ ... 50/26.html
shows a typical (reference design) 6850 draws more power than a 5770.http://www.techpowerup.com/reviews/HIS/ ... 50/28.html
shows the 6850 as about 30% faster in gaming than a 5770.
But given that a quick check of newegg shows the 6770 is cheaper than the 5770 I'd say the price comparison should be between the 6xxx cards.
cheapest 6770 $115
cheapest 5770 $140
cheapest 6850 $155
All of that is of course ignoring noise. It'll cost you more effort and/or money to get a very quiet card. Depends on your noise floor if it's worth the money/effort.
oh and since newegg lists them the fanless options are
fanless 6750 $120
fanless 6770 $140
but the heatsinks are very large on those cards so check to see if they will fit in your case.
Oh and the one that newgg doesn't have is
fanless 6850 $215 see google products PowerColor SCS3 AX6850 1GBD5-S3DH
why they made recommendations like that was because of how the pricing was. Pricing changes, recommendations should as well. Check benchmarks in your favorite games, check prices on your favorite retailers and make a choice. Don't let a 6 month old stale review point you to a card that isn't a good buy any more.