Advice on a new rig

[MiRai]

Using ISBoxer to multibox.

Were you using a super-custom Window Layout with Swap Groups or Instant Swapping disabled? If not, then all of you clients have most likely been rendering at 2560x1600 the entire time. It's not really a big deal, but in order to give a better estimate of hardware requirements I'd have to see your profile to know exactly how you have your layout set up.

When in SLI does VRAM "double" as well or is it just the raw GPU power that is "doubled"?

No, the VRAM is mirrored; and rarely do the GPUs scale perfectly when in SLI.