The 260fsb is real & then x2 =520 on a DFI Infinity.
Advantage of lower voltage is as has been said
1) can run cooler & quieter
or
2) because you are .2V ahead of the game you can overclock like mad
bit noisy though.
As to the benefit of running 2.6-7GHz it depends what you do with your PC.
If you are doing something CPU intensive then obviously an extra 1/3 cpu speed helps, if you are browsing/word-processing then nothing noticeable.