In deciding on building a new system I select a processor/motherboard family (eg. the LGA 1155 0r LGA 2011) and video card (eg. GTX 670 or AMD 7750) and whether I am going to SLI or Xfire to make a determination about the PSU required. RAM does impact power requirements, as does number and sort of HDDs, but these are minor considerations. I want my maximum power requirement to be met and over supplied by at least 15%-20%.
I always buy quality products, and even if a product is new, I look for user feedback to make a final determination. In PSUs I try to stick to good quality OEMs like PC Power & Cooling, Seasonic, Antec, Corsair, OCZ, inter alia. Generally you weant a quality PSU with a single rail 12V design.