http://ifirobotics.com/first-legacy.shtml
That website is going down soon, you'll have to look to the Vex Wiki when it does.
You need to know if you have a 2004, 2005, or 2006-2008 RC.
Differences:
2004 and 2005 have a smaller processor from the 2006-2008.
2004 has a different OI from the rest
2007-2008 had different radios too (giant things compared to the old ones), but all 2000-2008 radios will work if combined to a matching pair (e.g. red text radio to red text radio, blue text radio to blue text radio, giant 2007 radio to giant 2007 radio).
There is camera code in some of the 2005-2007 code. Beware of that. There are versions without it. If you have a 2005 RC, you might run out of code space if you are using some crazy autonomous programs or need to stuff alot of code on it (although this is really really unlikely)