I also have a old ProLiant 4000/6000 rack. Is 5.5 what I need, too?

Yep. Any non ML/DL/CL server will not work properly with 6.0 or above. And certain ML/DL/CL ones also don't support the 6.0 CDs. Basicially, if the server uses F9 to get to a BIOS setup type program, it can use the 6.0 CD. If it uses the F10 partition, it needs 5.5.