By Kasper Jade and Katie Marsal

Published: 09:00 PM EST

Apple Computer is believed to be prepping one final update to its PowerPC-based PowerBook G4 product line that could be unveiled at the end of next month, AppleInsider has learned.

Reliable sources say the PowerBook update is slated for an introduction during the third week of September at Apple Expo Paris, but has recently fallen into a rut that could force a delay or even cause the company to scrap the revision entirely.

Two new models that have shown up on radar are referenced as the PowerBook5,8 and the PowerBook5,9. Both Apple's current 15-inch and 17-inch PowerBook G4 systems identify themselves as the PowerBook5,7, with the 12-inch model being listed as a member of the iBook family (PowerBook6,8).

At the high-end, the new PowerBooks are expected to gain a mere 30MHz speed boost that would put the 15-inch and 17-inch models at 1.7GHz. Additionally, the new 15-inch PowerBook is expected to step up to the feature set of the current 17-inch PowerBook.

The new PowerBooks are also expected to be the first Macs to gain support for dual-channel DDR2 SDRAM, which can reach higher frequencies with less energy consumption than standard DDR SDRAM.

At least one of the two new models will also gain a higher resolution (or higher density) display, sources say. Rumors that Apple was preparing to offer higher density displays with its PowerBook line first surfaced in February, after the company accidently shipped a user manual for its 17-inch PowerBook referencing an unannounced model with a native resolution of 1920 x 1200 pixels.

Screens with a resolution of 1920 x 1200 pixels are categorized as WUXGA displays, which is short for Wide Ultra eXtended Graphics Array. PowerBooks currently ship with WXGA (or Wide XGA) displays, of which the 17-inch model sports the highest resolution at 1440 x 900 pixels.

The new PowerBooks will reportedly gain software support through a special build train of the yet-to-be-released Mac OS X 10.4.3 Update, which remains in development.

Precisely what will become of Apple's 12-inch PowerBook offering is an unknown. Prior to Apple's announcement to switch to Intel-based processors in 2006, reliable sources said Apple planned to abandon its 12-inch PowerBook offering with the release of the current model. It's unclear if these plans have changed in light of the Intel announcement and the fact that all PowerBook G4 models will see no further revisions after this year.

Still, at least one factor threatens to hamper Apple's plans to release the new PowerBooks next month, AppleInsider has been told. Specifically, it's believed that Apple's PowerPC development team has been met by a shortage of resources as the company shifts the vast majority of its engineers to its forthcoming Intel-based projects.

The new PowerBooks are expected to be powered by Freescale's new MPC7448 PowerPC G4 processors. The MPC7448 is based on Freescale e600 PowerPC core, and boasts a 1MB Level 2 backside cache, which is double that of the 512KB cache included with Freescale's MPC7447A chips used in the current PowerBooks.

The MPC7448 is also the first product in the MPC74xx family to use Freescale's 90 nanometer (nm) silicon-on-insulator (SOI) CMOS process that will significantly increase clock and bus speeds while reducing power requirements.

The PowerBook G4 line is one of Apple's two best selling Macintosh products as of late, with the iMac G5 being the other. By producing one more PowerBook G4 revision this fall, Apple will be able generate added interest and maintain higher average selling prices moving into the holiday shopping season, said a respected Wall Street analyst who asked not be named. But it would not be detrimental to the company if it failed to do so, the analyst added.

Apple chief executive Steve Jobs is currently scheduled to deliver the keynote presentation during the September Apple Expo in Paris.