How often do you upgrade?

Earlier tonight I was at a friend's house. She was having a hard time browsing certain websites, she told me (no, not those sites), and wanted to know if I could figure out why. I knew the answer as soon as I walked into the computer room, of course. The copy of Internet Explorer 5.0 she was running on her 500mhz G3 iMac via OS 9.2 was a little long in the tooth. When I told her that I believed Explorer was the issue (in fact, the site she was visiting even posted a "please upgrade your browser" message when loaded), she said that she had been just thinking about upgrading her machine. Another friend of mine just bought a USB-to-802.11b bridge for his G3 iMac, because original Airport cards (and those adapters they require for use with those older iMacs) are hard to come by and pricey.

This made me wonder: When do people upgrade? I bought a 333mhz G3 iBook in 2001 (I think) that I got rid of maybe two years ago. My main machine at home is a 1GHZ G4 flat panel iMac. I still love it, it does what I need it to do (Warcraft is a little choppy at times, but not intolerable), and I have no intention of replacing it in the near future. Of course, I'm extremely tempted by the brand new iMacs, but I'm the type who keeps the old car running past 100,000 miles, just to see how long I can keep it on the road. It's a pride thing.

So, TUAWers, when do you upgrade? Are you the type who opens a fresh new box every three years, or is OS 8 still rocking your world? Discuss.