Originally Posted by Monster Pete

Personally, I wouldn't bother unless the parts are already worn out. Chainrings tend to last a bit longer than cassettes, which in turn tend to last longer than chains. Just replace parts as they wear, unless you're looking specifically to change the setup. BTW, a derailleur will generally shift whatever number of sprockets you ask it to as long as the cable pull is compatible with that of the shifter, so there probably won't be a need to replace that even if you're changing the sprocket count.

Some good reference articles here:
http://sheldonbrown.com/speeds.html
http://sheldonbrown.com/cribsheet-spacing.html