According to some of the docs for legacy systems on the Compaq website,
the VAX 6000-600 is listed as having a 1GB memory limit.

  http://www.compaq.com/alphaserver/vax/archive/vax6000_600.html

I'm not sure how accurate this is though, since they screwed up the info
on the VAX 6000-400 which should be 512MB memory limit with a per CPU
rating of ~7 VUPs, not the 3.8 listed.  They also fail to mention that it
supports up to two vector processors.  It basically looks like they've
duplicated the specs from the VAX 6000-300.

  http://www.compaq.com/alphaserver/vax/archive/vax6000_400.html

Also of note, as can be seen on the main VAX page here,

  http://www.compaq.com/alphaserver/vax/archive/

they mention the VAX 8974 and VAX 8978 systems.  I'd never heard of these
boxes before.  Has anyone here ever seen one in the wild?  From the specs
it sort of looks like they were sold as clustered systems, 4 x VAX 8810
for the VAX 8974 and 8x VAX 8810 for the VAX 8978.

The VAX 7000 and VAX 10000 are listed as being able to support up to 3.5GB
of RAM.  The VAX 6000-600 seems to be a bit of an oddball.

-brian.