Cgminer itself does not use lots of ram indeed, however you DO need that large amount of ram just to create the buffers to speak to the GPU, even if they don't show up as used ram.
It isn't a question of the RAM the mining software uses, but the memory required by the algorithm the coin uses. SHA-256 (Bitcoin) does not require a large amount of memory to hash efficiently, while Scrypt (Litecoin) does.
You guys are right. I set up two dedicated rigs today taking all the cards off my main rig and put 4GB RAM on each. The rig with 6 7970s would not mine all cards. It would only do 2-3 at a time. I was worried something broke. I even re-imaged the operating system. Then what you guys said occurred to me. I put 8GB on the big rig and now all 6 cards can mine again. using scrypt.
8GB will do 6 7970s using tc-8192 with full speed hashrate, which is perfect since that's all that is needed on these cards. But just for test, I noticed I couldn't go higher than 8192 like I was able to do on the machine with 16GB. Just something for anyone else to keep in mind. Mainly with lots of 7950s I know you guys use high thread concurrency, how much ram are you using with high thread concurrency and 5-6 cards? More than 8GB?
Very awesome