As most people will tell you, they do "complete" rebuilds every X years and "updates" every X months/1 year.
For the pcs I advise (my personal and friend's pcs), I seem to be in the habit of new complete rebuilds every ~2-3 years (major processor revisions) with a ram "max out" after 1 year, and new or "upgrade to SLI/Crossfire" every year.
My recent personal history:
1. AMD Thunderbird / GeForce 2 Ti (Geforce 6800)
2. "One of last" Pentium 4 / Radeon x600 (No video upgrades on this one, as system 3 was built 2 years after this one due to this system's use of AGP vs. PCI-E)
3. Core 2 Duo / Geforce 8600 GT (2nd 8600 GT, 9800 GT w/ single 8600 GT PhysX)
However, due to using a slowly failing Nvidia MB in system 3, it will not be getting another graphics upgrade. I am pricing for a "major rebuild" (keeping Sata drives, disk drives, case, and ... until video/ram upgrade 1... the PSU) for mid-next year. (Following, hopefully, Nvidia's next release of cards.)