You absolutely do not need more RAM to hash... I never run more then 4GB of ram on a rig, and I average over 900KH/s/card at scrypt(1024,1,1) and 450KH/s at scrypt(2048,1,1) on my R9 290s. The "more RAM is needed" thing is pure unfounded bullshit.
Is that an absolute statement? Will it always apply?
If someone is only interested in mining scrypt or vertcoin (At its current N factor), I will say you are correct. Vertoin's N factor will eventually change (at the same pace as glaciers move - across land)
On other coins, that increment their N factor regularly, some are up to N=14. If you get more than 2 cards that run 4GB each, you'll want to have 8 GB system ram. The miner software has to allocate the scrypt buffer in system memory to start, and allocating a 3.5 GB buffer for 3, 4, or 6 cards just doesn't fit into 4 GB of RAM.