A lot of the carbon stems are infact carbon wrapped alloy stems as such unless you want to invest in a true carbon stem which is not cost effective you do not have any of the advantages carbon has to offer. Omega Pharma Quickstep use the Zipp SL145 full hand wrapped carbon stem on their Venges which retails in excess of £150
Carbon seat posts can reduce vibration from the road, however this depends upon how they are constructed.
A lot of pros still run alloy bars as carbon road bars are said to reduce feedback from the road. TT bars by contrast can be shaped quite specifically as such these lend themselves to carbon fibe characteristics.
Don't always believe that carbon equals lighter this is not the case, there are some damn heavy components out there.