just to give you small comparison number:
The system consumes ~ 45W when all harddrives are idle. Without the harddrives and tv-card i was able to drop the system below 20W (i think that is really an amazing number for a quad core system).
The hds are only used for 1-3h a day so they are set to spin down after 30minutes. i know that it is probably not the best for the drives as start/stop cycles are more dangerous than running all the time but i guess 1-2 stop cycles a day won't break them that fast. Usually i tend to keep them until the bigger drives get cheaper but since the flood that plan hasn't worked out.
If you plan to get a good cpu with low power consumption you might take a look at the new ivy bridge cpus from intel as they are pin compatible to sandy bridge and even use less power.
By spinning down the drives and using a smaller energy hungry cpu you could save some power. In my area 270W running 24/7 would be nearly 500€ / year just on energy costs.