Even at Apple they're not very sure about the exact processor speed they'll be able to see the new Mac at. What's for sure is that the processors will be PPC 7470 processors (G4), although Apple might have a low-end G4 tower using the 7460 chip (which sports no Level 3 Cache).
DDR SD-RAM is for sure, Apple will stress that the G4 is the only processor that can really make use of it.
We _might_ see a high end tower with two 1.5 GHz chips, but the sources *I* tend to listen to still seem to be thinking of the following line-up:
LE: Single 1.2 GHz 7460 PowerMacintosh G4
ME: Dual 1.2 GHz 7470 PowerMacintosh G4
HE: Dual 1.33 GHz 7470 PowerMacintosh G4
This would, of course, enable the iMac to hit the GHz barrier. It would also enable iBooks to use the Sahara G3 chip of up to 1 GHz, but the 'books will get updated only at Sybold this Autumn.