Phenom can do more per clock cycle. In other words, a 2.6GHz Phenom would be a lot faster than a 3.2GHz Athlon 64 X2.
If you're thinking about getting a Phenom, you might want to hold on to your money for a little bit longer. AMD's 45nm Deneb Phenoms are on schedule for Q4 2008, and they're looking pretty impressive.