Memory of graphics card is totally irrelevant past 2GB and just a marketing trick unless you're planning on gaming on three monitors at the same time. Notice that the cheaper card is no-name, while the more expensive one is actually some known brand. That's the price difference.
I'd suggest the following changes to
http://www.ibuypower.com/Store/Gamer..._E810/w/133841
Bottom line will be $10 less and you get better computer:
-Switch processor from i5-3570 to i5-3570K for +$20 - this will allow you to overclock later if you want to
-Switch processor cooling from Corsair H60 to Certified... for -$54
-Switch graphics card from Radeon 7870 to Radeon 7870, just one row up for no-name card to save $52
-Check on games from "free stuff" list
-Switch motherboard from Asus P8Z77-V LX one step up in the list for ASRock Z77 Extreme4 for +$41 ... this gives more upgrade options later
-Switch power supply from 800W standard to 650W Corsair TXV2