Don't forget to look out for "80 plus" certified PSUs - they tend to be more efficient than those that are not, especially at load. PSUs will obviously only fuel as much power as all the components in your PC will need, and not 'drown' it in power.
Here's also a rough guideline of what the connectors mean, let's take a Corsair HX650w PSU for example:
http://www.newegg.com/product/product.aspx?Item=N82E16817139012
1 x Main connector (20+4Pin) - Goes directly to the motherboard
1 x 8-Pin EPS/ATX12V - This is for the CPU
8 x Peripheral - Also known as 'Molex'. Some fans might need this.
9 x SATA - For all your hard drives and optical drives (also SSDs ofc). Most PSUs will supply you with quite a lot of SATA connectors.
2 x Floppy - For your grandma?
4 x PCI-E - To power up your graphics cards. Most only require 1-2, with the exception of going Crossfire/SLI.