ugrading + pc case + power supply

how often do u guys reuse the same PC case and power supply when u upgrade your pc ?

im thinking about going safe and buying a 500-600 power supply and nice big quiet case for my 1st built pc so i can reuse the components ...

your thoughts ?

reusable, assuming:
1.the case isthe size you will need for future mobo sizes
2. 500-600w of power doesn't become standard/obsolete
3. the psu doesn't burn out
4. it has the right hookups for future, undevelopedhardware

otherwise, sure