The 830MB RAM limit has been removed, but just like any 32-bit OS without bad tricks, GNU Mach can not cope well with lots of memory. Latest versions of the Debian gnumach package will limit themselves to 3 GiB of memory. If you want more, you can twiddle the VM_MAX_ADDRESS limit between kernelland and userland in i386/include/mach/i386/vm_param.h, but glibc and the hurd servers will not cope well with less than 1 GB.

There is a 64-bit port in progress.

If you have an older version, or still experience problems with vmstat (see above) reported much less memory than you have, the best is to limit the memory it can see via GRUB's upppermem feature. Add uppermem 786432 to GRUB's Hurd entry in menu.lst.