Acho que qualquer sistema com systemd consumirá mais RAM que um que use systemv, openrc, etc, não sei se devido a dificuldade em gerenciar serviços, ou outra coisa, o que sei é que no Debian Jessie removi o systemd, instalei o pacote de compatibilidade systemd-shim e os pacotes (sysvinit, sysvinit-core e sysvinit-utils) e voltei para o velho sysvinit.
E também acho (tenho quase certeza! rsrs) que qualquer sistema amd64, consumirá mais RAM que um sistema i386, i486, i686-pae (meu caso), observe como roda suave um OpenBox num Debian i686-pae:
Com o Opera 12.16 aberto = 583.4 MiB:
[17676:17665 0:14] 09:08:31 Qui Mar 19 [edps@debian: +1] ~
(1:14)$ sudo ps_mem.py
Private + Shared = RAM used Program
56.0 KiB + 18.5 KiB = 74.5 KiB unlinkd
136.0 KiB + 43.0 KiB = 179.0 KiB gnome-pty-helper
156.0 KiB + 25.0 KiB = 181.0 KiB sh
172.0 KiB + 31.5 KiB = 203.5 KiB vnstatd
192.0 KiB + 36.5 KiB = 228.5 KiB init
212.0 KiB + 57.0 KiB = 269.0 KiB cron
276.0 KiB + 50.5 KiB = 326.5 KiB grep
284.0 KiB + 90.0 KiB = 374.0 KiB dbus-launch
436.0 KiB + 27.0 KiB = 463.0 KiB ssh-agent
136.0 KiB + 366.0 KiB = 502.0 KiB saned (2)
156.0 KiB + 463.5 KiB = 619.5 KiB ip.sh
376.0 KiB + 247.0 KiB = 623.0 KiB menu-cached
404.0 KiB + 230.0 KiB = 634.0 KiB getty (3)
416.0 KiB + 339.0 KiB = 755.0 KiB gvfsd
580.0 KiB + 236.5 KiB = 816.5 KiB accounts-daemon
516.0 KiB + 314.5 KiB = 830.5 KiB gvfsd-metadata
660.0 KiB + 182.0 KiB = 842.0 KiB wget
668.0 KiB + 200.5 KiB = 868.5 KiB sudo
592.0 KiB + 278.5 KiB = 870.5 KiB gconfd-2
628.0 KiB + 436.0 KiB = 1.0 MiB gvfsd-trash
1.0 MiB + 256.5 KiB = 1.3 MiB polkitd
812.0 KiB + 511.0 KiB = 1.3 MiB dbus-daemon (2)
944.0 KiB + 410.5 KiB = 1.3 MiB ntpd
1.0 MiB + 428.0 KiB = 1.4 MiB gvfs-gdu-volume-monitor
1.4 MiB + 73.0 KiB = 1.5 MiB udevd
1.2 MiB + 339.5 KiB = 1.5 MiB console-kit-daemon
824.0 KiB + 772.5 KiB = 1.6 MiB udisks-daemon (2)
1.1 MiB + 730.5 KiB = 1.8 MiB lightdm (2)
2.1 MiB + 63.0 KiB = 2.1 MiB rsyslogd
1.8 MiB + 370.5 KiB = 2.1 MiB xbindkeys (2)
1.8 MiB + 361.5 KiB = 2.1 MiB compton
2.5 MiB + 29.0 KiB = 2.6 MiB dhclient
2.1 MiB + 533.5 KiB = 2.6 MiB bash
2.0 MiB + 778.0 KiB = 2.7 MiB mount.ntfs-3g (5)
2.4 MiB + 405.0 KiB = 2.8 MiB gnome-keyring-daemon
2.6 MiB + 961.0 KiB = 3.5 MiB cupsd
3.8 MiB + 885.0 KiB = 4.7 MiB wicd-monitor
2.9 MiB + 2.5 MiB = 5.4 MiB conky (2)
4.9 MiB + 624.5 KiB = 5.5 MiB wicd
5.0 MiB + 962.0 KiB = 5.9 MiB notification-daemon
4.9 MiB + 1.1 MiB = 6.0 MiB openbox
4.4 MiB + 2.1 MiB = 6.5 MiB leafpad
6.4 MiB + 2.8 MiB = 9.3 MiB lxpanel
8.0 MiB + 3.2 MiB = 11.2 MiB wicd-client
18.3 MiB + 4.7 MiB = 23.0 MiB /usr/bin/x-term
26.6 MiB + 420.5 KiB = 27.0 MiB squid (2)
27.3 MiB + 1.2 MiB = 28.5 MiB Xorg
30.4 MiB + 5.1 MiB = 35.5 MiB opera:libflashp
369.8 MiB + 2.4 MiB = 372.2 MiB opera
---------------------------------
583.4 MiB
=================================
Com o Opera 12.16 fechado e alguns serviços não essenciais parados = 141.2 MiB
[17676:17665 0:14] 09:09:07 Qui Mar 19 [edps@debian: +1] ~
(1:14)$ stop squid
[....] Stopping Squid HTTP proxy: squid[....] Waiting..................[ odone.
. ok
[17676:17665 0:15] 09:10:16 Qui Mar 19 [edps@debian: +1] ~
(1:15)$ stop vnstat
[ ok ] Stopping vnStat daemon: vnstatd.
[17676:17665 0:16] 09:10:27 Qui Mar 19 [edps@debian: +1] ~
(1:16)$ stop saned
Stopping SANE network scanner server: saned.
[17676:17665 0:17] 09:10:33 Qui Mar 19 [edps@debian: +1] ~
(1:17)$ stop ntp
[ ok ] Stopping NTP server: ntpd.
[17676:17665 0:18] 09:10:51 Qui Mar 19 [edps@debian: +1] ~
(1:18)$ killall conky
[17676:17665 0:19] 09:11:03 Qui Mar 19 [edps@debian: +1] ~
(1:19)$ killall xbindkeys
[17676:17665 0:20] 09:11:59 Qui Mar 19 [edps@debian: +1] ~
(1:20)$ uptime
09:12:27 up 1:37, 2 users, load average: 0,03, 0,88, 1,47
[17676:17665 0:21] 09:12:27 Qui Mar 19 [edps@debian: +1] ~
(1:21)$ sudo ps_mem.py
Private + Shared = RAM used Program
136.0 KiB + 51.0 KiB = 187.0 KiB gnome-pty-helper
192.0 KiB + 44.5 KiB = 236.5 KiB init
212.0 KiB + 66.0 KiB = 278.0 KiB cron
284.0 KiB + 132.0 KiB = 416.0 KiB dbus-launch
436.0 KiB + 31.0 KiB = 467.0 KiB ssh-agent
376.0 KiB + 272.0 KiB = 648.0 KiB menu-cached
404.0 KiB + 264.0 KiB = 668.0 KiB getty (3)
416.0 KiB + 379.0 KiB = 795.0 KiB gvfsd
580.0 KiB + 259.5 KiB = 839.5 KiB accounts-daemon
516.0 KiB + 356.5 KiB = 872.5 KiB gvfsd-metadata
592.0 KiB + 308.5 KiB = 900.5 KiB gconfd-2
668.0 KiB + 238.5 KiB = 906.5 KiB sudo
632.0 KiB + 481.0 KiB = 1.1 MiB gvfsd-trash
812.0 KiB + 530.0 KiB = 1.3 MiB dbus-daemon (2)
1.0 MiB + 284.5 KiB = 1.3 MiB polkitd
1.0 MiB + 464.0 KiB = 1.4 MiB gvfs-gdu-volume-monitor
1.4 MiB + 85.0 KiB = 1.5 MiB udevd
1.2 MiB + 370.5 KiB = 1.5 MiB console-kit-daemon
824.0 KiB + 813.5 KiB = 1.6 MiB udisks-daemon (2)
1.1 MiB + 790.5 KiB = 1.9 MiB lightdm (2)
2.1 MiB + 74.0 KiB = 2.2 MiB rsyslogd
1.8 MiB + 557.5 KiB = 2.4 MiB compton
2.5 MiB + 37.0 KiB = 2.6 MiB dhclient
2.0 MiB + 820.0 KiB = 2.8 MiB mount.ntfs-3g (5)
2.4 MiB + 476.0 KiB = 2.9 MiB gnome-keyring-daemon
2.9 MiB + 168.5 KiB = 3.1 MiB bash
3.9 MiB + 454.0 KiB = 4.4 MiB cupsd
3.8 MiB + 986.0 KiB = 4.8 MiB wicd-monitor
4.9 MiB + 651.5 KiB = 5.5 MiB wicd
5.0 MiB + 1.2 MiB = 6.1 MiB notification-daemon
5.2 MiB + 1.4 MiB = 6.6 MiB openbox
4.4 MiB + 3.0 MiB = 7.3 MiB leafpad
6.7 MiB + 3.7 MiB = 10.4 MiB lxpanel
8.0 MiB + 3.8 MiB = 11.8 MiB wicd-client
19.2 MiB + 5.5 MiB = 24.8 MiB /usr/bin/x-term
23.5 MiB + 1.4 MiB = 24.9 MiB Xorg
---------------------------------
141.2 MiB
=================================
[17676:17665 0:21] 09:12:40 Qui Mar 19 [edps@debian: +1] ~
(1:21)$
A antiga dica abaixo é de minha autoria (era né já que removi o perfil):
http://www.vivaolinux.com.br/dica/Obtendo-informacoes-sobre-o-uso-dos-recursos-do-sistema
Um abraço.