Just in case I get involved in one of these...
...here's how I understand the current standings (DP 800 vs AthlonXP2000+/Pentium 4 2.2GHz. Mac OS X.12 for the mac, WinXP for the PCs):
OS stability: Both good, not sure which is better right now
OS ease-of-use: undecided, I like OSX, but some people hate the dock, think it's slow, etc... WinXP has some problems like slower internet, driver signing, activation, etc...
OS power: Mac, command line, SMP, UNIX programs, plug-and-play clustering, built in Apache, etc...
OS Appearance: Mac (Obviously)
OS Performance: PC, all that OSX eye candy has a price.
Free software: Mac, iPhoto, iDvd, iMovie, iTunes, etc... Wins by a lot.
Performance (using PC133, DDR, and RDRAM respectively): For photoshop and other Altivec enabled tasks the G4 wins hands down, for anything else I think the AthlonXP 2000+ wins by a bit. For sheer performance (instead of performace per dollar) I think the 2.2GHz Pentium 4 wins by a very small margin on non-Altivec tasks (source: Tom's Hardware did a very nice comparison review of the 2.2GHz P4 and the AthlonXP2000+)
Hard Drive: equal, 80GB 7200rpm is about as good as you're going to find on a PC.
RAM: PC, PCs are starting to come with 512MBs of DDR or RDRAM, compared to 256MBs PC133 in the G4/
Graphics Card: PC, the GeForce 2mx is a low end card in the PC world. GeForce 3 or better on a high end PC.
Optical Drive: Mac, PCs with anything like a superdrive are very rare and expensive.
Ports: Mac, PCs only rarely have firewire
Floppy Drive: Mac, floppies are out of date.
Networking: Mac, Gigabit ethernet, AirPort.
Clustering: Mac. AppleSeed makes it very easy, someone recently got 233GFlops on a 100Bt ethernet G4 cluster and said that their results implied no intrinsic limit on the size of a Mac cluster.
Software Support: PC, by a tiny bit. Most of the additional software PCs have is junk, but they have some specialty apps.
Hardware Support: PC, custom PCs are such a big market that there's tons of stuff out there for them, Macs aren't bad though.
Reliability: Mac
Customer Support: Mac
Hardware Appearance: Mac
Price: about the same, leaning towards PC. If you get a PC with equivalent hardware and software (including a nice case), it's roughly the same.
Value: Mac, roughly the same price, lasts longer.
Feel free to correct, disagree with, or add to this list.