i just buy a new computer, i still have my old gaming computer as a file server, i get a new computer pretty much every year (staggering laptops and desktops)
I just upgraded to an LGA1366 system after having used a Socket 478 Pentium 4 system since early in 2003.
I've never upgraded just the motherboard. I try buy a high end system that will last for several years and upgrade disk and memory if necessary during it's lifetime.
I'm upgrading now to move to a 64-bit OS and get beyond the 2GB memory limit of my old system.
Well, let's see. Last summer, I finally pitched my 12 year old P233MMX and it's motherboard. My workhorse machine is an eight year old Barton core AMD system.
However, I am planning to move everything to my Gigabyte Q6600 system. Except games. Those are on my OC'd Q9550 system.
I usually, not always, but usually buy a new motherboard with a processor.
Not that I really have to, but by the time I want a new CPU, I am also ready for a different motherboard as well. I do it about every year to year and a half.