Depends on what it's being used for. My collection, which has a variety of parts (SGI, Sun, PPC, and a virtual x86 env) gets used for testing and learning. I don't buy hardware that I don't have a purpose for anymore - I guess you could call that moving out of the "ALL THE SERVERS" phase. Next things I'd like to add (but not due to cost) would be a recent HP-UX server (I have no issue buying the software from HP, it's the hardware that's expensive) and AIX (once again, recent POWER hardware is more then I'm spending right now). Knowing the UNIX families well translates to money in the bank (the Sun stuff certainly does, and I've seen postings for good money doing AIX. I know a few places that are still using IRIX, would love to see one of them put a posting up).
I've also contributed patches to open source projects based on testing against some of the hardware I have access to