Switching from the G5 processor to Intel chips is a huge step forward. There is one wrinkle, however.
All Mac software -- from Apple as well as from independent developers -- must be rewritten to run on the Intel processor, or must use a translation program called Rosetta*. Rosetta - the technology that allows existing PowerPC software to "just work" on Intel-based Macs - is the missing link that makes this transition possible. (although emulator seems be be a more apt description, Apple seems to prefers the term "translator").
The newly rewritten software will get a noticeable speed boost, while older Mac software running through Rosetta will be somewhat slower.
Going forward, Apple will emphasize a new software specification called Universal that will eventually make Rosetta unnecessary. Software built under Universal will automatically install a version designed for Intel chips when loaded on an Intel Mac, and a version for PowerPC when loaded on an older Power PC Mac. More about Universal.

*Name derived from Rosetta Stone