aborted: that may be true (initial cost comparisons). How many years with the PC in question last? Speed-wise, how comparable are the machines when running real-world, cross-platform benchmarks? i.e., Photoshop filter tests (fair ones, not like Apple's filter set, which is heavily mac baised), cinebench, lightwave renders?
I have found that while PCs are cheaper (on average), in order to get one with comperable functionality and speed, the machines are not much cheaper (Apple does not currently offer a low-end $400 machine. Dell offers a top of the line Intel systems which will beat a G5 on most tasks, but they are also often more expensive than the G5). See the ARS battlefront for the endless price comperisons RE: Dell/Apple.
Plus, IME, x86 based machines have a significantly shorter lifespan than do macs. I have a AIO mac setup as a mail server right now. It's about 7 years old, and runs the most recent revision of OS X.
One advantage to x86 systems? you can build you own middle-range linux machine for significantly less than a Mac, and for the price, get a new one every 2-3 years. However; that requires the ability, time and desire to build such a machine. I don't always want to build a new machine every 2 years.