For a "Home" PC an Intel Core 2 Duo(above 2.0 GHz) or Intel's I3 processors are more than capable for surfing the web, casual gaming, watching movies, etc. Granted the I3's are more expensive but are better.
I would use at least 3GB's of ram for a Core 2 Duo system and i would say 4GB's for an I3 System since i have never seen any I3 on any system with anything less than 4GB.
Windows 7 Home Premium is sufficient but make sure to have a 64 bit Operating System when dealing with 4GB or greater of ram because a 32 bit Operating System wont use more than approximately 3GB.
For the Power Supply i would use one with at least 400W to keep all the components powered.
You could go for low-end Cards like an Nvidia Geforce 9400 GT which is good for 1080p video and okay for casual games. Note: this wont play high demanding games like Battlefield 3 or Crysis at respectable resolutions or frame-rates.
Also Note: that the Core I3 processor has an integrated graphics card embedded in the processor so a graphics card for a system with this processor is not necessary. MAKE SURE THE CORE I3 YOU CHOOSE HAS AN INTEGRATED GRAPHICS.
Also include a basic DVD Drive
Also you will need a PCI card for internet unless the motherboard supports LAN or Ethernet cables. You could also get a WLAN card for internet if the consumer wants wireless networking.
I Hope this helps as a start!