I might be wrong, but I think it's about RAM on the graphics card, and that's part of the reason why Scrypt-N11 performs at half the hashrate of Scrypt-N10 on the same hardware. I'm running my rigs with the same 4GB as always, it doesn't seem to require any more system RAM.
But no, all people talk about having 8 gigs of ram.
Well, they can talk all they want. Maybe Windows needs that much, I don't know. This is BAMT with 4 cards and vertminer:
Mem: 2830160k total, 1320704k used, 1509456k free
PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
20 0 591m 162m 67m S 3 6.0 25:58.63 vertminer
As you can see it's even less than 4GB total, I probably forgot to disable Intel graphics.