A PC is just a tool, what do you plan to use yours for?
Hard core gaming with 3 monitors and a SSD drive for super fast load times?
Video processing with 8TB storage?
Home theater entertainment center?
Email, web surfing, youtube, skype?
Each use determines which CPU, how much memory, graphics card requirements and how much power you need. Use a calculator to make sure you have enough:
http://images10.newegg.com/BizIntell/tool/psucalc/index.htmlSave $10-$20 with newegg's cpu & motherboard combo deals.
And if your existing power supply will work double check to see if it has the right number of connectors for your motherboard and graphics card.