Another reason for going only to 12 GB now, is that when 8GB modules become affordable, you can, if needed, jump up to 24GB by replacing the two 2GB modules at that time, keeping the two 4GB modules you just bought.
Eventually, you can replace these, taking it to the max of 32GB.
Software, over time, demands more and more RAM, so while 24 or 32 may sound insane today, tomorrow the need may be there, and it's good to have a plan