A beautifull, lightweight, highly customizable X-system monitor – Conky. Works on my Xfce and LXDE Xubuntu install.

Add to Start-up Applications “$ conky -d”, which in Xubuntu will be located here: ~/.config/autostart/conky.desktop

[Desktop Entry]
Encoding=UTF-8
Version=0.9.4
Type=Application
Name=Conky Monitor
Comment=System Monitor
Exec=conky -d
StartupNotify=false
Terminal=false
Hidden=false

My ~/.conkyrc (borrowed mostly from a post on ubuntuforums.org): :

background yes
use_xft yes
xftfont HandelGotD:size=9
xftalpha 0.5
update_interval 1.0
total_run_times 0
own_window yes
own_window_type normal
own_window_transparent yes
own_window_hints undecorated,below,sticky,skip_taskbar,skip_pager
double_buffer yes
minimum_size 200 5
maximum_width 200
draw_shades no
draw_outline no
draw_borders no
draw_graph_borders no
default_color white
default_shade_color red
default_outline_color green
alignment top_right
gap_x 12
gap_y 48
no_buffers yes
uppercase no
cpu_avg_samples 2
override_utf8_locale no

TEXT
$sysname $kernel on $machine

Uptime $alignr $uptime
Load $alignr $loadavg

Hostname $alignr $nodename
eth0 $alignr ${addr eth0}

Inbound $alignr ${downspeed eth0} kb/s
${downspeedgraph eth0}
Outbound $alignr ${upspeed eth0} kb/s
${upspeedgraph eth0}

$processes processes ($running_processes running)

CPU $alignr ${cpu cpu0}%
${cpubar cpu0}

MEM $alignc $mem / $memmax $alignr $memperc%
$membar

swap $alignc $swap / $swapmax $alignr $swapperc%
${swapbar}

/ $alignc ${fs_used /} / ${fs_size /} $alignr ${fs_free_perc /}%
${fs_bar /}

/home $alignc ${fs_used /home} / ${fs_size /home} $alignr ${fs_free_perc /home}%
${fs_bar /home}