You wouldn't need 6GB. The latency on main memory for a GPU is horribly bad so bad that any process which needs random access to GPU main memory will be annihilated by a CPU in terms of performance. GPU memory is designed to stream textures and as such it couples massive bandwidth with extreme latency. Scrypt was designed to fill the L3 cache of a CPU. The developers of alt-coins had to intentionally lower the memory requirements by 99% to make GPU competitive. Yes Litecoin and clones use about 128KB of cache. The MINIMUM memory requirement for Scrypt is 12MB. It doesn't take 16GB. Try it out yourself or check out various hacking forums the OpenCL performance for Scrypt (2^14, 8, 1) is beyond pathetic. A cheap CPU will run circles around it.
This begs the question: Was this known to the devs of Litecoin and/or Tenebrix? I mean, why else did they intentionally lower the memory requirements? Big scam after all (hurr durr gpu resistant)?
I have never got a satisfactory answer. I will point out that it is intentional though. The default parameters for Scrypt are (N=2^14, r=8, p=1), the parameters used by Litecoin are N=2^10, r=1, p=1).
I am not sure if was a scam but the end result is the same, Litecoin is 99% less memory hard then the default Scrypt and about 1/7000th as memory hard as the parameters recommended by the author for high security (not realtime) applications.