http://www.cracked.com/blog/using-windows-7-may-lead-to-murder/

-Get a good case with good cooling - this is the a very good way to extend the life of your parts

-never ever skimp on a powersupply, get more power then you think your parts need and make sure its a good brand / 80+ (there are also different classes of 80+ see http://www.80plus.org/manu/psu/psu_join.aspx)

-make sure the ram you are getting is on the motherboard's QVL (qualified vendors list) and/or from a trusted brand. You can always find the QVL on the manufacturers website along with downloadable versions of the mobo's manual.

-never buy an nvidia motherboard (with nForce chipset) (lol venn)

-wait for an upgrade in gfx card generation and buy a previous gen card (shortly after new cards come out) which is still relatively new for ~60% of the original price (this happens 2-3 times a year)

ncix is your friend

most considerations come from what price point you are trying to hit and what you are going to use the computer for