I think that G4 apps will come out pretty soon, because Exec SG allows it, and in the XE version the price difference between the g3 & the g4 is so small than a G4 seems to me like a much better option.
We are switching to ppcs because we need raw power, the Altivec unit will allow us to do number crunching way better than integer units... that means for example creating divx way faster than using integer/floating point instructions...
the 750FX may be faster with some operations, but I think that in the long run a G4 will be much better.