Buying the right computer

-- How important is the HD's RPM when it comes to loading time? Installing? Recording a video of something you're doing via a software like Fraps?
-- How important is it to match the Power Supply Unit's output with your computer components' needs? What happens if you go over/under the amount required?

• may give you some idea: http://www.tomshardware.com/reviews/sas-...,1702.html
  but I would stick to the major product line. If you simply want to increase data transfer speeds, opting for higher RPM drives is not worth the cost.(at least atm)
  
• use a power supply calculator like this http://educations.newegg.com/tool/psucalc/index.html then +10~20%, also considering the further upgrade you may have
  if over amt required: you will spent too much on the PSU and electric bill, more pollute to the environment....
  if under the amt.: system may become unstable