I'm slowly upgrading components in my main rig one at a time and saving those parts to assemble into a 2nd rig. I had to replace my mobo when my first one needed an RMA, so when I got it back from MSI I kept it as my 2nd S939 platform. I just recently picked up an Opteron 165 to replace my A64 3500+, so I'm saving it for the 2nd rig. My next plans are to upgrade my GPU, then my RAM, and along the way I'll pick up some HDD's and optical drives, and then finally a case and PSU.
Sure, I'm committed to the S939 platform for a while, but with a new GPU and better RAM to go with my OC'd Opty 265, I think it will do everything I need it to do for a couple more years. In the mean time I'll use the old parts to build a media center PC.