When hd6850 was launched it was bit slower than hd5850 but with driver improvements performance between them is almost same but hd6850 consumes lesser power relatively.
HD7770 performs close to hd6850 but consumes a lot lesser amount of power than hd6850.
HD 5850 is selling for around 10k+ and is older gen so not worth it.hd6850 is a better option than hd5850.
hd 6850>=hd 7770>hd 6790>hd6770~hd7750>hd6750>gt640(ddr3)
In 6-7k range the cards to get are hd6770 and hd7750.Between 8-9k the cards to look for are hd6850 and hd7770
The choice of card depends how much you can spend. If you can spend 9K + then just get HD 6850 and Corsair CX 430 V2. This will also be a future proof solution because currently HD 6850 can play games well 1080P and in future there is ahigh chance of playing demanding games @ 1366X768 resolution with good settings with this card. It also performs better in Tessellation heavy games compared to HD 6770 ( a rebranded HD 5770 with 3D support).
Hi. I own a HD 6850 2GB and it is bloody awesome. Gives me 400 to 500 fps constantly though I limit it to 250 in CoD4. I'd say it's an awesome card for 8k and I'd suggest that card to you as it is worth it. Very worth it. I play all games on ultra high settings and it is just lovely.