What is the point of a 4gb graphics card? (GeForce GTX 680)

Ok, im trying to build a computer to buy and i never have a computer good enough to play the total war series (referring to Empire and most definitly Rome 2). I am also going to start playing Armas' and i might download 3d videos and play it on a 3d 55inch tv. I figuring just the stuff on the gaming side though, so i figure i need atleast a GeForce 670 for a Good (+) graphics card. Having this already in my mind saw the Geforce 680 4gb single card. Would i be better off to get the 670 or the 4GB 680? i saw a whole different array the next day i looked to like a Geforce 680 6gb single,double, and triple card. i dont even know what to think about that **it.

so what im asking is what is the 680 4gb even used for is there some game in the world that actually needsit or that people buy it for a specific purpose for?
Why?

Heres my specs i have on a computer as of today ( ive been modifying parts down for a better price kindof)




Intel X79 Core i7 Configurator
1 x Case ( Corsair Vengeance C70 Gaming Case - Gunmetal Black )

1 x Case Lighting ( Cold Cathode Neon Light - Blue )

1 x Processor ( Intel® Core™ i7 3930K Processor (6x 3.20GHz/12MB L3 Cache) - Intel Core i7 3930K w/ Intel Performance Tuning Protection )

1 x iBUYPOWER PowerDrive ( PowerDrive Level 1 - Up to 10% Overclocking )

1 x Processor Cooling ( Liquid CPU Cooling System [SOCKET-2011] - ARC Dual Silent High Performance Fan Upgrade (Push-Pull Airflow) )

1 x Memory ( 32 GB [4 GB X8] DDR3-2133 Memory Module - Corsair or Major Brand )

1 x Video Card ( NVIDIA GeForce GTX 680 - 4GB - Single Card )

1 x Video Card Brand ( Major Brand Powered by AMD or NVIDIA )

1 x Free Stuff ( [FREE In-Game Voucher] - $150 Value In-Game Currencies: Hawken, Planetside 2, and World of Tanks ($50 for each title) - Free with purchase of NVIDIA GeForce GTX660 or above )

1 x Free Stuff ( [6-Port] NZXT Internal USB Expansion System + Bluetooth & Wireless N Modules - FREE w/ Desktops over $1299 )

1 x Motherboard ( ASUS P9X79 LE -- 2x USB 3.0 )

1 x Power Supply ( 650 Watt - Corsair CMPSU-650TXV2 - Free Upgrade to 750 Watt Corsair CMPSU-750TXV2 ($20 Savings) )

1 x Primary Hard Drive ( 1 TB HARD DRIVE -- 32M Cache, 7200 RPM, 6.0Gb/s - Single Drive )

0 x Data Hard Drive ( None )

1 x Optical Drive ( [14X Blu-Ray] LG BLU-RAY Re-Writer, DVD±R/±RW Burner Combo Drive - Black )

1 x Meter Display ( NZXT Sentry 2 Touch Screen Fan Controller & Temperature Display )

1 x Sound Card ( ASUS Xonar DG )

1 x Network Card ( Intel Pro 10/100/1000 Network Card )

1 x Operating System ( None- Pre-formatted Hard Drive Only )