剛查了一下Promise網站FastTrak SX4000的datasheet
最高可加到256MB SDRAM,最少需64MB才能工作,沒有錯

Simply add up to 256MB SDRAM cache memory for enhanced
data I/O handling.* FastTrak SX4000 uses an onboard, hardware-based
XOR engine to rapidly calculate parity data for RAID 5 arrays. The
32-bit / 66MHz PCI interface provides extra bandwidth for data
transfers up to 266MB per second.
*Requires minimum of 64MB for operation