I haven't played Riddick, but I remember FarCry looking amazing after installing my 6800GT.
IIRC it was the first game I tried and it looked stunning! Can't remember the exact settings I used back then but I know it was 1600x1200, probably with low Antistropic Filtering though, but it still ran smooth.
If you wanna waste £50-70 getting a few extra frames per second (that you can't see) to boast about, even though the GT can do Ultra speeds, then by all means do so, but it doesn't mean that everyone should do the same.