> Each httrack_cmd takes 400M in RAM just as it
> loads.

Httrack loads the links in the cache into ram. My big mirror has 44k links
(15k html files) httrack takes about 150MB.
Your mirror must be a very large site.

If you don't need everything, filter or limit the depth. If it's forum, you
don't need showuser, reply, print, etc., and you don't need different
different presentations chronological, threaded, etc.

You might also try combining those into one mirror (multiple urls/mirror)