Actually, it depends. ATI/AMD generally do it so that you add 900 to the current generation to get previous generation performance. 4850+900=5750:http://www.anandtech.com/bench/Product/177?vs=173
And it works, the 2 are quite close. So, doing that, 5850+900=6750. Now, RUMOR has it, however everybody has said this rumor, so there may be some truth to it, that 67xx will have 256 bit GDDR5, just like the 5850 and 5870. The 6770 releases on October 13th as you may already know, and therefore the 6770 or 6750 might compete with the 5850. Once again, RUMOR has it that they will be priced $180-250, which makes decent sense, but it might be higher. In any event, it might be wise to buy a 6xxx series card instead. Although look at the 4870. It started out at about $300, similar to the 5850, and after a while, dropped to about 200$ once the 4890 came out, and now its at $140 after several months of being below the 5xxx series. All in all, my guess is they might drop it to $200-230. But this is entirely guessing, speculation, and rumor, best just to wait till the 13th and see for yourself.