I use 27GB of RAM just for the disk, this is the minimum to fit all of WoW's MPQ files.

Each WoW client takes up near 800-1GB. As I type this I'm running 10 clients and using 38.5GB system ram totel. I figure you're going to need 48-64GB ram to make it work without troubles.

I went with a Corsair set - DDR3 1866Mhz 8x8GB for about $415 on newegg. There are 8 slots in the x79 mobo I have, quad channel, maximum memory is 64GB. I couldn't find 16GB consumer grade RAM anywhere so 128GB or 64GB with only 4 slots was not happening.

I'm not really sold on 2100+ RAM at the moment given its cost and reviews I've read about how little difference it made in anything other than benchmarks and I'm not a benchmarker, I'm a multiboxer. So that being said, RAM is RAM is it's going to be 10x faster than your SSDs in Raid either way and considerably faster than an HDD. My raptor 10k runs like molasses compared to the SSDs, but it's for larger storage.

You can do what some have done is find the files that WoW hits the most and make a smaller RAM drive for those only, 8-16GB or so just to take some of the load off. I don't know how well this works though.