2012 new iMac 27.5 in i7 32GB 1600MHz DDR3 SDRAM - 4x8GB or 16GB?

I am buying the new iMac 27.5 in i7 and I can not decide if it's best to spend the extra $400 on the 32GB 1600MHz DDR3 SDRAM - 4x8GB or to stick with the 16GB and CAN I buy more RAM at a later time? I have an old 2009 iMAC and I did buy extra RAM and installed it myelf from website distributor. Can this be done to the new iMAcs? Can RAM be added later? The people at Apple tell me no but I'm not sure if that's them in sales wanting me to buy the max on everything. Thanks for any help out there!!!

Jake