Для таких объемов ОЗУ - только LXDE более или менее будет работать. Во-первых, часть отсюда хорошо помогает:
http://vasilisc.com/speedup_ubuntu. Во-вторых, выпиливайте все ненужное: compiz, cups (если нет принтеров и не планируется), hplip, evolution (thunderbird вместо него), empathy, bluetooth, onboard, gnome-mag, network-manager (через консольные утилиты настройка), avahi, ненужные драйвера...
ubuntu-desktop apport apport-gtk apport-symptoms python-apport update-manager update-manager-core update-notifier update-notifier-common software-center aisleriot gbrainy gnome-sudoku gnome-games-common gnome-mahjongg gnomine quadrapassel at-spi python-pyatspi onboard gnome-orca gnome-mag gnome-accessibility-themes libgnome-mag2 brltty brltty-x11 python-brlapi libbrlapi0.5 espeak espeak-data libespeak1 speech-dispatcher python-speechd libspeechd2 avahi-autoipd avahi-daemon avahi-utils libnss-mdns libavahi-core6 libavahi-gobject0 libdaemon0 bluez gnome-bluetooth gnome-user-share libgnome-bluetooth7 bluez-cups bluez-alsa bluez-gstreamer obex-data-server obexd-client libopenobex1 pulseaudio-module-bluetooth byobu computer-janitor computer-janitor-gtk checkbox checkbox-gtk dnsmasq-base network-manager network-manager-gnome mobile-broadband-provider-info network-manager-pptp network-manager-pptp-gnome libnm-glib2 libnm-util1 ppp pppconfig pppoeconf pptp-linux wpasupplicant wireless-tools modemmanager empathy empathy-common nautilus-sendto-empathy nautilus-sendto libpurple-bin libpurple0 telepathy-salut telepathy-idle telepathy-haze telepathy-gabble telepathy-butterfly python-telepathy python-farsight python-papyon telepathy-mission-control-5 libtelepathy-farsight0 f-spot fglrx-modaliases bcmwl-modaliases friendly-recovery gwibber gwibber-service hplip hplip-data hpijs foo2zjs pxljr libhpmud0 cups-driver-gutenprint min12xxw libgutenprint2 foomatic-db foomatic-db-engine foomatic-filters openprinting-ppds pnm2ppa splix system-config-printer-common system-config-printer-gnome system-config-printer-udev ibus ibus-m17n ibus-table ibus-gtk python-ibus libibus1 m17n-db m17n-contrib libm17n-0 nautilus-share pcmciautils pitivi radeontool simple-scan sane-utils libsane tomboy toshset transmission-gtk transmission-common tsclient vinagre vino libgtk-vnc-1.0-0 rdesktop telnet ttf-indic-fonts-core ttf-kacst-one ttf-khmeros-core ttf-lao ttf-punjabi-fonts ttf-takao-pgothic ttf-thai-tlwg ttf-unfonts-core ttf-wqy-microhei ubuntuone-client ubuntuone-client-gnome rhythmbox-ubuntuone-music-store libubuntuone-1.0-1 python-ubuntuone python-ubuntuone-client python-ubuntuone-storageprotocol launchpad-integration python-launchpad-integration python-launchpadlib usb-creator-common usb-creator-gtk xserver-xorg-video-all xserver-xorg-video-apm xserver-xorg-video-ark xserver-xorg-video-ati xserver-xorg-video-chips xserver-xorg-video-cirrus xserver-xorg-video-geode xserver-xorg-video-i128 xserver-xorg-video-i740 xserver-xorg-video-mach64 xserver-xorg-video-mga xserver-xorg-video-neomagic xserver-xorg-video-openchrome xserver-xorg-video-r128 xserver-xorg-video-radeon xserver-xorg-video-rendition xserver-xorg-video-s3 xserver-xorg-video-s3virge xserver-xorg-video-savage xserver-xorg-video-siliconmotion xserver-xorg-video-sis xserver-xorg-video-sisusb xserver-xorg-video-tdfx xserver-xorg-video-trident xserver-xorg-video-tseng xserver-xorg-video-vmware xserver-xorg-video-voodoo xserver-xorg-input-all xserver-xorg-input-wacom xserver-xorg-input-vmmouse xserver-xorg-input-synaptics w3m