The Athlon X2 6000+ doesn't come at 2.6ghz, that's the 5000+.
But even if you had the default X2 6000+ the X2 7750 is still 15% faster clock per clock to the Athlon (because its a Phenom 9750 with two cores disabled).
Its not much of a performance difference though.
In short: 6000+ < X2 7750
5000+ < X2 7750