I don't mean to pull people from this site because this site is very very helpful but there is a HUGE database of RROD Repair and diagnosis information that would take months even years to accumulate here.
The [http://www.xbox-experts.com|Xbox Experts Site] even has a step by step guide to diagnose your xbox and then a list of possible fixes.. I have repaired over 50 xbox's with this and had great results.