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.

I have 7 gigabyte R9 290s in 2 windows rigs. You MUST run 8gb RAM to get them going full speed. I had 4gb and they limped along. You can't turn up the TC or I without issues otherwise.