I didn't use the Dell installed Ubuntu 8.04 for long, so I can't comment on it. But my Mini 9 runs Ubuntu 9.04 pretty reliably. I haven't rebooted in over 9 days. (Knock on wood
Usually system lockups are caused by lousy drivers, try updating your system ... if possible.
Also, the next time it happens, try pressing "Ctrl+Alt+Backspace" to restart the GUI (this will kill all your programs, too
That command might be disabled by default, you could also try "Ctrl+Alt+F1" to change to a simple text terminal instead of the GUI. Then login and execute the command "sudo /etc/init.d/gdm restart && exit" to restart the GUI.
"sudo" runs a command with superuser privileges, "/etc/init.d/gdm restart" is the command to restart the GUI, and "&& exit" just automatically logs out of the terminal when the GUI is successfully restarted.
If neither "Ctrl+Alt" command works, you'll have to take more forceful action, like holding down the power button.