Upgrading to the new CPU will not require you to purchase a new motherboard if you are already an Athlon user, however you will have to perform a BIOS upgrade. Initial test seem to reveal quite a powerful little performer, especially for games.
We tested a reference system from AMD configured with the new chip; 1GB of DDR400 memory; a 10,000-rpm, 74GB Western Digital hard drive; an NVIDIA 6800 Ultra graphics card with 256MB of RAM; and Windows XP Professional. The unit earned a score of 116 on WorldBench 5, which ties for the second highest score on this benchmark with a previously tested AMD reference system using a 2.4-GHz dual-core Athlon 64 X2 4800+ chip (the rest of the configuration is identical). It also bested the 107 average score of two previously tested systems with the Athlon 64 FX-55, as well as the 102 score of a reference Intel system using the 64-bit 3.73-GHz Pentium Extreme Edition chip.
Reviews from around the web: 3dvelocity, Hot Hardware, Xbit Labs, The Tech Report, bit-tech, HardOCP, ExtremeTech, and Hexus.