It's always worth it to upgrade from Windows Vista. Upgrading to the 6xxx series will definitely make an improvement. How much depends on how much you're willing to spend. The 6750 isn't a bad card at all, but you won't be able to run it max settings. For that, you'll want a 5970 or 6950.
I should add that I don't have any problems with Vista. I really have zero complaints about it. I would upgrade if I knew it would upgrade on top of Vista. I don't wanna wipe my computer out if I don't have to. But as far as graphics, is DX11 that much better that it's worth it to buy windows 7?
Why is a 5970 better than a 6750? I just figured the higher the number, the better the card.
I wouldn't really say that DX11 in and of itself is worth it, but Windows 7 is more streamlined and less of a resource hog, and benchmarks have shown that games running on Windows 7 and XP perform about the same, while Vista is a bit behind. You can still upgrade your hardware first and make a decision about wiping later.
As for the numbers, here's the naming convention:
The first number is the series number. This changes approximately every year I believe. The 6xxx series is newer than the 5xxx series.
The last three numbers is where you can pick the higher number and it will be better. The reason a 5970 is a good choice is because while it is last generation, it was the top of the line card back then. Because it's last generation, it'll generally be a better value.