I am in the process of building a new PC and have ordered most of parts from
http://www.newegg.com.
A mouse and keyboard are for the most part pretty much standardized either PS/2 or USB, once you know what type they are it is just a matter of getting what you like. That being said, I would suggest taking a hands-on approach to both the mouse and the keyboard by checking them out a local store, friend, or some other means. make sure they fell comfortable to you prior to buying them. It could save you some money and pain in the long run.
My current build is a water cooled Intel i7 920 processor on an Asus P6T6 WS Revolution motherboard with 12GB of G.SKILL DDR3-1600 RAM, 2 1TB Western Digital hard drives, 3 XFX GTX285 video cards (also water cooled) a Samsung Blu-Ray reader/DVD-RW, Thermaltake 1200W power supply in a Mountain Mods Extended Ascension Case running MS Windows Vista Ultimate 64-bit edition.
I have all of the parts except a few of the water cooling parts.
Sure its overkill, but I am just doing my part to stimulate the economy.