RAM REQUIREMENTS ON-MOTHERBOARD
The equivalent amount of system RAM as for the sum of all the vRAM used by the GPUs is required when mining with reaper.  For instance, 3x 5970s mining with thread_concurrency values of 8000 each would require 3x 500MB = 1500MB system ram (plus additional RAM for the OS).  The memory requirements for 7xxx cards are also higher because of larger thread_concurrency values, for instance a thread_concurrency of 24000 means 1.5GB system RAM per card is required.

You shouldn't need a lot of RAM for cgminer, it offloads it to the GPU unlike reaper which for some odd reason keeps it in the system RAM.  reaper is coded really sloppily while cgminer is pretty immaculate.