Depends on what you want to do with your Mac. Email, Word, Safari, iTunes will all run perfectly well in a single 1.8 G5 PM. Where you're going to see a difference is in apps like Final Cut Pro 4, Photoshop, and other dual-proc aware apps, or if you like to watch DVDs while playing UT2003 and downloading bitTorrents as your iMovie renders, iDVD encodes, and Folding helps the world. 😉
Seriously, though, you'll only notice a slight difference if using many apps at the same time. The big difference is in the pro apps rendering.