1) Freezes upon heavy IDE access between two different drives.
2) Freezes upon heavy USB -> drive access (large color scans).
Memtest showed nothing as well. The other two solutions I've considered:
1) Needs two banks of RAM, not one. Apparently there are some issues with the nVidia built-in card conflicting with memory access.
2) Replace the built-in video card with a stand-alone.
Have you actually replaced the memory with the XMS so that you know the solution is good?