I have been active in the Bitcoin scene since it was publicly announced, i have 250 bitcoins (if i can retrieve my wallet), and i am as large of a supporter of bitcoin as i can be but i am not here to ...
From reading charts, it seemed like the 5850/5870 are the only cards that push 400 Mhash for a decent price. The 6850 is rated around 300, not bad. The 5970 is up around 800Mhash, but is not cheap.