The G4 is a PowerPC chip, developed by IBM, and Motorola, and produced by Motorola. It's advantages lie in the altivec engine, which allows fast vector computations (ie. 4 computations at once, instead of 1). This is a huge advantage over the G3 in altivec apps. The whole processor is marginally better than the G3 at similar clock speeds. The P4 is, well, an Intel chip. You won't be running Mac OS X on it, and that's reason enough to stay away. It's main advantages, though, are HT (HyperThreading) which allows something similar to altivec to be accomplished. Any computer you buy right now (possibly with the exception of the iBook) is going to be fast enough for whatever you need to do.