For me, In my opinion, because PC parts are all electronic gadgets that needs to be tested (for a certain number of hours or period of time), It is better to buy them all within the period of the product's warranty. For me the only parts that are safe to buy first is the casing as long as it can support up Long graphics card and Extended ATX like the HAF 912.
But besides the casing, everything must be tested, from speakers, monitors, keyboard and mice, motherboard, processor,GPU, PSU, RAM,HDD, DVD writer etc.
And from what I know, even though how great a brand is, for example ASUS mobo or Gigabyte MOBO, sometimes there is a "BAD APPLE" among the bunch, that is why it needs to be tested as soon as possible after purchase =P