Here is a chart comparing the Workgroup Server 6150, 7250, and 8550.
Product Features 6150/66 7250/120 8550/132
--------------------- --------------- --------------- ---------------
Processor 601/66 MHz 601/120 MHz 604/132 MHz
L2 Cache 256 KB 256 KB 512 KB
CPU Card No No Yes
RAM (shipping with) 16 MB 16 MB 24 MB
RAM (expandable to) 128 MB 256 MB 512 MB
Number of RAM Slots 2 4 8
VRAM DRAM video 1 MB (2) 2 MB (2)
Number of VRAM Slots n/a 4 4
Expansion Slots 1PDS/NuBus 3 PCI 3 PCI
Drive Bays 3 4 4 (1)
Internal HD 1.2 GB 1.2 GB 2 GB
Internal CD Player AppleCD 600i AppleCD 600i AppleCD 600i
Internal Floppy Drive 1.4MB SuperDrive 1.4MB SuperDrive 1.4MB SuperDrive
Internal DAT Drive n/a n/a DDS-2 DAT Drive
System SW System 7.5.1 System 7.5.3 System 7.5.3
OT 1.1 OT 1.1
Configurations Two Three Four
(1) The Workgroup Server 8550 has four drive bays set up by Apple. An additional "fifth" drive bay can be added to the lowest bay, but the drive cage and cable have to be provided by a third party and installed by an authorized Apple service technician.
(2) VRAM DIMMs must be 112-pin fast-paged mode, 32 bit wide, and 70 ns RAM access time or faster. Do not use 256K VRAM SIMMs.