The 8-core option is as an upgrade option to the current quad-core 3 GHz model, for a total of 24 GHz of computing goodness. There are no other 8-core options, and the rest of the lineup has not changed (although they've dropped the prices on the Cinema Displays).
There has in fact been a small change to the options. The smallest hard drive available now is 250 GB. Previously, the smallest available was 160 GB (although the standard configuration was 250 GB).
The other interesting thing is the fact that nobody else seems to have these 3 GHz quad-core Clovertown (X5365) chips, which Apple says it's using.
Opt for the 8-core Mac Pro and you get the power of two Quad-Core Intel Xeon “Clovertown” processors running at 3.0GHz. Or choose a quad-core Mac Pro featuring two Dual-Core Intel Xeon “Woodcrest” processors and decide how fast they fly: 2.0GHz, 2.66GHz, or 3.0GHz. At 3.0GHz, the quad-core Mac Pro runs up to 2x faster than the Power Mac G5 Quad.Intel had originally suggested that these chips wouldn't be out until Q3 2007, but this is nearly a full quarter early, and Apple says the machines are shipping within 3-5 days.