I'd say open the wow folder.
Find the file wow.exe
Double click it.

Repeat 2 more times.

So you have 3 copies of wow open.

See how the game runs, without IS running.
I'm sure IS uses ram too, but not a whole lot of it.
So it will be marginally worse with IS too.

But you can at least see if your system can handle 3 instances of wow.

It's not specifically vista that allows more then 3gb of ram.
You can have a lot of ram on XP 64-bit, Vista 64-bit or Win7 64-bit.