Well benchmarks should be done with a beastly CPU and almost always are. It's all about what the GPU can do assuming there is nothing in the way.
It's not like the 1GB is entirely for textures, that's absolutely all the GPU has to work with, everything else including any form of AA takes memory as well. The last time I cranked up the textures in a game it took almost 2GB.
System memory and video memory are used for different tasks and are generally not shared which is why the PS3 was limited. Sharing any of the system memory would pose issues considering it's pretty much always slower than VRAM. The 360 used GDDR3, the VRAM, for the entire system.