Case, PSU, optical drive, fans, mouse/keyboard, speakers and monitors change pretty rarely.
Slightly more frequent changes in RAM, SSD/HDD, OS and heatsinks but they are still relatively 'safe'
Motherboards, CPU's and GPU's tend to change quite a lot and are large parts of a system's overall performance so they are less 'safe'
More specifically; Ivy Bridge, GTX 600/HD 7000 series and the newer SSD's (Vertex 4/Intel 520 etc.) are all quite new so it should be at least a year or so before any major changes.