changing a process' priority...

from the gui:

Run the system monitor as root with

gksu gnome-system-monitor  

Then under the "view" menu move the radio button to "All processes"

Then you will see all users processes and can change their priority.

from the command line:

First find your processes PID (Process ID) with pidof like so:

pidof name-of-process  

It will return the PID of the given process, usually a four to five character number. Then run

renice priority processes-PID  

Replacing priority with the priority you want (-20 being the highest and +20 being the lowest) and process-PID with the PID you got earlier.

renice is used to change the priority of a running process, if you want to start a process and change it's priority you use nice.

nice priority name-of-process

Replacing priority with the priority you want and name-of-process with the name of the process you want to start.

Note: In order to give a process a negative priority you must have root access.