I would say if you don't have enough RAM, then RAM is your cheapest upgrade.
And lacking enough, also your best upgrade possible.
Once you have enough, more doesn't do you much.
And you might need a 64-bit OS, to use the ram otherwise having it is pointless.

Processor is massively important for Warcraft.
Almost all of the video effects are more processor intensive than video intensive.
If you have enough ram for your clients, this would be the biggest upgrade.

Hard Drive upgrades from a platter drive to a solid state drive is a huge upgrade too.
And given the relative cost of a low end SSD, versus a processor upgrade that might require a motherboard and ram as well...
Per cost, an SSD might be a larger improvement... but this is more for loading times, zoning times, etc... not nearly as much of an impact as the processor once you're in the zone and playing.

Personal experience may vary.