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.