Not too sure on this.  I have read conflicting things.  I read you need to have 1/2 the ram that you do video memory.  I have rigs with five 7950s and 8 GB DDR3 but they only use about 2 GB total.  So really I dunno.  But ram is so cheap these days I wouldnt worry about it.  If you end up needing more just go buy some.  Its cheap.