I don't think the issue really lies with big textures.
I barely use any (less than 128MB maybe) in my game, have near unlimited RAM and VRAM and yet it still crashes with DirectX errors, right now more than ever (which is not a bad thing for debugging).
There is something wrong with this engine I fear, I wish it was a user problem.
EDIT: The error:
Error E2005: Can't create DirectX texture unnamed
D3D_TexCreate: 8007000e - E_OUTOFMEMORY
How is "unnamed" set (or not set), could you, Mr. Lotter, use that field to add debugging information?