The differences in 10.4 running on G4 and G5 processors is subtle and not earth shattering. It is basically exactly the same code but it has little parts of it that take advantage of the differences between the two processors.
10.4 running on a Duo Core processors is a completely different kettle of fish. It is written for a completely different type of processor and cannot be run on the G4 or G5 processors. This version of the OS is a "Universal Binary" application which means that it is processor aware and will run, natively, on either the IBM processor or the Intel one. As you install it, the correct code for the current processor is installed. All of the built-in software that comes with a new Mac as been re-written, as Universal, so that it will run natively on either processor.
For those programs that have not been converted, Apple are using a piece of software called Rosetta. Named after the famed stone, I guess, this app converts the old IBM type code into the new Intel type on the fly and, by doing this, allows programs that have not been converted to run. As they run in Emulation mode, they are slower than those that run natively. Without Rosetta, none of the existing applications will run under the Intel Processor. This is rather like the change from Motorola to IBM PowerPC processors where apps had to be re-written, an emulator was used here too, and again in the move from OS 9 to OS X. Apple has, for some considerable time, been encouraging developers to use their X11 tools to develop/convert their apps. Using Apple's tools, it is possible to code once and only make minor changes for different platforms. A checkbox, checked, will apparently produce a Universal Binary app that will run on the Intel processors. Tools like Code Warrior are not capable of compiling in the required manner. Adobe, as you have discovered, are dragging their heels but have moved up the possible release date of CS3 to an earlier time.
CS2 will run at full speed on existing IBM processor Mac's and I'm pretty sure that Adobe will be offering an upgrade path from 2 to 3 when they get their act together.
Hope this helps a little