You might find this article about titled G4 vs K7
an interesting read. The K7 uses a lot of RISC-based architechture, as does the G4. In fact this site has quite a bit of good info about various CPU's.
Personally I'll take a PIII over either one any day - very over-clockable (my 450 runs at 558), extremely stable at high speeds, and much lower temp than the AMD chips... Never experienced a G4, and I don't really plan on it (two OSes are enough for me, thank you