First off, looks like you've got a solid start with that build. CPU-wise, an i7 would be significantly better, particularly since hyperthreading offers a noticeable performance improvement in multithreaded programs like CAD environments. That being said, the issue of whether it's worth the extra money depends more on your other uses and budget. The FX is a decent processor which should be more effective in coming years as more programs are optimized for more cores. In general, NVidia GPUs are better options for CAD than AMD due to their CUDA cores (don't know a whole lot about the reasoning tbh, but I've heard it a number of times from friends who CAD.) The equivalent of a 7870 is a GTX 660, but a 760 is a good choice as well. The 670 you mentioned is fine, but 700-series cards have better performance/price. On a side note, the power supply you have listed seems questionable; if you get a 550W, make sure it has a high efficiency rating and choose a well-known brand like Corsair. Overall, the lifespan depends on intended uses as well. If you're gaming in addition to CAD, a GPU upgrade every few years is never a bad idea. If you do CAD only, I don't see any reason you couldn't get 5+ years out of it without needing upgrades.
Anyway, good luck with the build.