Well, all your textures are also stored in the normal RAM.
DirectX has some pooling options for them, I also had alot of problems with acknex when my models/textures/maps were too big for it.
I never checked the real cause of this error, pretty sure its related to acknex and how it manages the RAM.
Maybe you should ask JCL about this? I would also be interested in an answer.
What you need to know is that you can have max 4gb RAM with acknex since its x86 (32 bit only)