IBM Power 550 Express (8204-E8A) Memory Upgrades

As defined on the System memory page, there are a few different memory options available:

Memory features
                                         Feature     Minimum    Maximum
Feature                                  number      quantity   quantity
=======                                  ======      ========   ========
1024 MB PC2-5300 (2 x 512 MB RDIMMs)      4520        0          16
2048 MB PC2-5300 (2 x 1024 MB RDIMMs      4521        0          16
4096 MB PC2-5300 (2 x 2048 MB RDIMMs)     4522        0          16
4096 MB PC2-5300 (2 x 2048 MB 1Gb RDIMMs) 4532        0          16
8192 MB PC2-5300 (2 x 4096 MB RDIMMs)     4523        0          16
16384 MB PC2-3200 (2 x 8192 MB RDIMMs)    4524        0          16

The general take-away is that memory must be installed in pairs, and the system can hold a maximum of 16x 'features', or 32x RDIMMs. (4x CPU modules, each with 8x RDIMM slots) The memory itself is standard DDR2 ECC Registered memory.