Not sure how well you would 10-box with that.

Warcraft would be physically limited by the amount of video ram you have.
If you don't have enough, then the game will not start at all.

After that, not enough ram is probably the biggest bottleneck.
And the cheapest fix, too.
I generally like to have 1GB for the OS, plus 1GB per client.
But I'm sure you could play with less than 1GB per client, particularly with minimal addons.

It can be a toss up between Processor and Hard Drive for the next upgrade.
A low end SSD can be an enormous boost in performance.
But the game is much more processor dependent than any other aspect of your system.



You can get trial accounts for free.
And while they cannot trade, use the auction house, whisper to anyone who does not have them on their friends list, etc...
They can have toons loaded on them, in SW/Org...
Which is a good indication of the stress level your system will face.

Grab a bunch of the trial accounts.
And see how many your system can handle.

You could also run Perfmon while doing this.
To get an idea what your bottlenecks are on the system.
Which are the ingredients you would ideally like to improve first.