Other than that all I can recomend is that you try out lots of different editors i listed here and find which one hits the sweet spot for you,
btw unless you have a quad don't expect to be able to do HD video
Most of the packages you mention (the non pro ones) are not SMP aware, as such the Quad will not be of much benefit over a Dual 2.5, 2.3 will only be slightly slower and a 2.7 will actually be faster.
Remember a Quad is only faster if the software in question is actually SMP aware and most non pro apps aren't.
970MP inside Quads has 1MB L2 Cache, twice as much as the 970FX in Dual 2.5, 2.3 or below, which actually makes a big difference. And even if the application is not multiprocessor aware, the rest of the system is, so a whole core can be used just for the video App, instead of using about 90% of it (the other 10% for system and apps in background).
I haven't tried it, but I though HD 720p (not Full HD 1080) should work in most Dual G5s. Even some heavily upgraded G4s can.
Yes that's true but the full benefit is with the Pro apps that can really stretch all four cores.
The 2.3 also has the 1Mb L2 cache (it's dual core not dual CPU).
Re the HD editing yes you can of course do this on a G5 (if you're prepared to be patient depending on the model) I did this a but on my 1.42 MDD (though the G5 was a much nicer experience)