Nah, the only thing that makes it better from a normal 6800 is the amount of memory. A normal 6800 has better memory clock, and about the same core clock.
And the difference between a 512mb card and a 256mb card ain't that much, sure the 512mb one is better, but not anything worth going crazy about.
U could get a card like 6800GS AGP for almost the same price.
doesn't have 512mb, but has almost double the memory clock etc.