I have 6 GBs of RAM and a GTX 570 on Win 7 so my RAM is capped at around 4GBS as far as the system can tell. I STILL can't use High Textures because of this problem, which I'm willing to call a bug, or an incredibly stupid decision on the devs part at this point.
Sounds like a bug or other issue. I also have 6GB RAM, and am using a GTX295 (which only has 850MB VRAM per GPU - much less than your GTX-570), and I have no problem with high textures.
