First, convert the texture files to .map files. Open a XSI command prompt, browse to the folder where the textures are, and execute:
imf_copy filename.extension filename.map
Do that for each texture.
If you have a Batch Univeral license, you can render it with the ray3.exe (mental ray standalone). Export the scene to a .mi2 file and render this file.
If you render through the GUI, check Enable Subregions and define regions size. The render will occur in separate parts.
Also try increasing the Job Memory Limit (Render Options > Optimization).
Hope this helps
Salutations - Cheers
Bernard Lebel