Need help building desktop for BF3 Ultra settings

I am in the market for a good Desktop for BF3 which can run on High to Ultra settings and I always want it to still be able to be used for college work, such as Microsoft Office. I am new to PC's and I have no idea what proper components I need but I have a friend who can get parts for 40-60% cheaper than websites and stores so I need help with what parts are needed so I can explain to him what I need and give specifics.

Approximate Purchase Date:January 2012

Budget Range:600-800

System Usage from Most to Least Important: Playing BF3 and other games, Microsoft office/college stuff, internet

Parts Not Required: keyboard, and mouse.

Preferred Website(s) for Parts: none, just need to know the good parts

Country: States

Parts Preferences: with good processor and an Nvidia or Radeon top of the line graphics card for ultra BF3 settings

Overclocking: Dont know what this is

SLI or Crossfire: Dont know what this is

Monitor Resolution: whatever is smallest, cheapest, and works fine with BF3 ultra settings

Additional Comments: Like I said, I can get the parts, I just need to know what will be the best parts for a great PC for BF3 and other high tech games, Microsoft Office, and surfing the web. ALso, I hear alot about doubling video cards for better performance. WHat does this mean and what advantage does two Radeon 6850's have over a Nvdia GTX560?