My main machine (other then getting a slightly bigger HD and more RAM) is pretty much the same as it was when I put it together new in 2001 (ancient). I keep looking at new tech thinking about getting a new machine, but outside of getting into new games (Starcraft 2 looks promising when it comes out) I just don't need one. Going retro gaming pretty much removed the need to upgrade my main machine every 2 years like I used to. Just about anything made in the last 5+ years will do web, irc, youtube, and office work. Sure a new system will rip DVDs faster but I download and burn them to DVDr to play on a real DVD player, not archive them and play them on a smaller PC monitor.
You might want to get a nice cheap/free B&W. Easy enough to put in a cheap SCSI card for all the old SCSI gear, takes cheap SDRAM and discarded IDE drives, plus it still has ADB plus USB and Firewire.
If you stay behind the bleeding edge long enough you get all the "obsolete" stuff for next to nothing or free even as others upgrade. Who uses SCSI, firewire, or IDE anymore, people junked all that working and useful gear for SATA and USB. If you like classic mac OS 7/8/9 you can find plenty of people tossing their pro apps out because they need OSX versions. Granted the new versions probably have some added functionality but for casual users you will never need it.