It used to be I'd get generationally huge gains with a new system every couple of years. Now it's only worth upgrading because something broke (AGP graphics, DDR RAM, processor on outdated socket/chipset, etc.) and it's not cost-effective to replace it anymore vs. moving to a new platform.
My now nearly 5-year-old (with a couple of cheap incremental upgrades) Athlon 64 desktop running a fully patched up Windows XP Pro still does everything I want it to.
I finally replaced an old Compaq Pentium III Mobile laptop last year because the screen blew up and the battery was starting to deflate. I see myself using the sweet Lenovo Thinkpad T61 that suceeded it for a long time to come.