The only reason to ever get a 5830 is if you feel brave enough to try to flash it to a 5870/5850.
However, I doubt the success rate would be very good.
In either case, the 5770 is a solid performer for it's price range. It is just about on par with a 4870, but will improve with driver maturity.
If you could afford the extra, a 5850 would offer a longer lifespan and would reduce the need of crossfiring it.