I think this depends on what sort of budget your on etc as recycling old parts and just upgrading a MB, CPU or GPU if possible tends to give you the most performance for your money.Your assuming that the PC its in is going to sacrifice it. It's the same argument as upgradable sockets, many people like me, build a whole new rig and the old one gets either sold or off loaded as a whole.
I've always found that if you want to skimp upgrading part by part you never get off the treadmill. I have always been a New PC every 4 years, maybe a graphics card swap mid life depending on the releases (of both games and GPUs)