The computers are serious speed daemons, with the ability to access 8GB of RAM, a fast 1GHz processor bus, SATA storage and up to two 2GHz CPU's processing data in 64-bit chunks.
Of course, they won't be available until August, but you can order them now. Apple also lowered the prices (and performance) of some existing PowerMac computers. Go to the Apple Store if you are interested.
Until they are in individual's hands, there will be some debate over the performance, but a series of developers, including Adobe and Mathematica were certain about the new duals knocking the socks off of the fastest P4 Xeon dual machines.