none of the links work...
1gb is exactly the same as 1024mb
higher amount of vram only helps when your gpu is fast enough to use that vram. That includes the bit rate and memory clock speed. In other words, your gpu needs to have a well balance of gb and bit rate.
the cheapest hd7770 ghz I can find is the xfx core edition $99 after rebatehttp://www.newegg.com/Product/Product.aspx?Item=N82E16814150598
But if I was you, I'd definitely get the sapphire vaporx one, its simply too gorgeous and has much higher core and memory clock speed not to mention the vapor x cooler design. although its about $25 more expensive but would perform about 25-30% better than the xfx one as well.http://www.newegg.com/Product/Product.aspx?Item=N82E16814102993