Basically, any PSU and optionally case that you buy now should last you through at least 3 motherboard/CPU/RAM swaps. I have a Thermaltake Xaser VI case that I've used for 9 years and don't see any reason why I can't use it for another several years.
The PSU (Cooler Master Silent Pro M2 850W) is only two years old because a power inversion in my UPS blew my last one (a nice Thermaltake that prevented any damage to any other of my PC's componenets, which blew me away, I figured my whole computer was going to be scrap). I would recommend either an M2 or V-series if you wish to go with Cooler Master.