StarCraft II recommends the ultra texture resolution for graphics cards with 1GB of video RAM and the high texture resolution setting for cards with 512MB of video RAM. We've found that even graphics cards with less memory can often handle higher texture settings without a notable performance penalty, so we've tested StarCraft II to see how much of a difference the amount of video RAM makes.
We tested the difference by benchmarking the graphics cards with less than 1GB of memory at the ultra setting and again using the high setting.
As you can see, most folks with a 512MB graphics card can use the ultra texture resolution without concern. There is a slight performance hit at resolutions of 1920x1200 and higher, but it isn't significant enough to negatively affect gameplay.