I have two r9290 based rigs. Both required the same amount of system RAM as the sum of the RAM in all the installed GPUs, or in other words, if you have 2 r9290s each with 4 Gb of RAM, you will require 8 Gb of system RAM. This is due to the usually high TC value the 290s  require to reach their full potential. All my 290s are hashing between 870 and 890.
In the other hand, Gigabyte R9 280x GPUs can get by with only one Gb of system RAM per GPU, due to the fact that those GPUs TC sweetspot is usually around 8192.