I would wait for the 7000 series cards. A newer generation, near 2x as fast (by the rumors), and will have much better performance per watt. The high-end cards will have Graphics Core Next Gen which will make it easier to develop general purpose processing and the high-end cards will use XDR2 DRAM which is twice as fast as the GDDR5 used in the 6000 series. There are LOTS of rumors and threads on the Southern Islands. The mid range cards may come in December and the high-end cards may come in January-February. The low-end cards are DEAD because of the APUs.
I'd concur - you should wait. $300 is much lower than 5970s have cost in the past, but compared to performance available for the same price it's nothing earth shattering. The next gen of cards will provide even more performance for each dollar spent and could leave you feeling like you got a bad deal on this previous-gen card compared to the new stuff.
The 5970 will be faster than your 6770's, but probably not enough to justify the money. I'd go for the 7xxx card that fills the $300-350 price point when it's released and then crossfire if you still need additional performance.
EDIT: or wait a few weeks (hopefully) longer after 7xxx release and take a look at NVidia's offerings, too.