"renice" in the terminal. use "top" to find the "PID", use the syntax "renice +1 <PID>". the +, one to twenty, decreases priority, make it "-1" etc for higher priority. I only know of one renice GUI app, called "BeNicer" on versiontracker...there are others, but benicer is similar to Activity Monitor.
Burning stuff, best not to change the nice value, you may get coasters. I'm sure encoding/decoding stuff for DVD shouldn't have problems with other priorities. Usually best to leave the prioritization to the OS, stability and all that, but sometimes the user knows what needs to be done faster or what doesn't need to be done.