IN day to day usage, I doubt you'd be able to tell the difference between a Pentium4 running WindowsXP and a G4 733 running OS X 10.1...
When doing video editing, the G4 might be slightly faster.
In dirty, hard work the G4 might take about half of the victories.
In Altavetc optomized apps, the G4 wins hands down.
In unoptomized/SSE2 optomized apps, the P4 destroys the G4 hands down.
The thing that you have to realize though, is that the two are champions of two completly different realms. CISC and RISC Or Windows/Apple's OS's. Apple's are nice machines, and they do somethings better than PC's. Alot of people are used to 'em, and will use nothing else. Just like I *will not* use a Mac because i've never used one and I don't know how. I don't care if someone offered me a dual 1GHZ G4 for 1500$, I'd rather have an AthlonXP 1800+ or a Pentium4 2.0A rather than a G4 running OSX. It's my choice, because I choose not to own a Mac, I need my gaming and my apps that are PC exclusive, and I have nothing I want to do on a Mac that I can't do faster on a PC.
For some people, it's the other way around. "I need final cut pro, I need the G4's speed with MPEG-2 encoding/decoding and I need OSX because I'm not used to WindowsXP." and honestly, these people I can understand. If I was them, I wouldn't even touch the PC architecture.
It all depends on what software you wanna use. I'd say that for day to day use you wouldn't notice the difference between a 2.0GHZ Pentium4 and a G4 733. Nor would you notice the difference between a 1.4GHZ Thunderbird and a 2.533GHZ Pentium4. Not in the apps that I use anyways. (Internet explorer/Word/Excel/Winamp/Emulators) So just tell him to respect your choice of the PC platform and tell him that you'll respect his choice of the Mac platform and you both can live in peace.