Nope, I mean the 5850. You have to remember that AMD did some dumbstuff scaling with it's cards so the 6850 would be comparable to the 5830 and the 6870 to the 5850.
The card you have now is more powerful than either of those. That being said, you can get a new 5870 on newegg for $200 which would probably be the fastest thing you could get in that limit.