The currently shipping version of OS/400 (now called i5/OS) is Version 5 Release 4
The machine models (it’s now referred to as the System i/5) are:
Model 520 (1-2 processors)
Model 550 (1-4 processors)
Model 570 (2-16 processors)
Model 595 (8-64 processors)
You can get a pretty complete understanding of the capabilities of the various models by referring to the draft Redbook titled “System i/5 Handbook” at this link:
If the link doesn’t work right just go to the www.ibm.com web page and put SG247486 in the search box and it should take you to the current edition of the redbook.