Utilising a similar architecture to the current socket 939, AM2 is to offer several new improvements. It will introduce a 940 pin socket design that can utilise faster DDR2 memory (up to 800MHz memory when paired with compatible Athlon 64 X2 and Athlon FX CPUs) and Nvidia's latest Nforce 5 chipset. The first processor to take advantage of this new socket will be AMD’s high-end dual-core Athlon 64 FX-62, which is also being released today along with the Athlon 64 X2 5000+.
Naturally, since AM2 is a new design, you will require a new motherboard to upgrade to AMD's Athlon 64 FX-62. Upgrading to FX-62 will give you a 2.8GHz core clock frequency, as well as a pleasing 2MB for each L2 cache.
All in all, Anandtech sums it up really well in a few words "Same Performance, Faster Memory, Lower Power". Other articles/reviews are available as well: TechReport, HotHardware, and ChileHardware (spanish).