IS Boxer actually taxes the video card, as if everything was rendered the same as the largest window.
It then scales down the image to fit the smaller window.
But strains the system, as if you had five large windows, each the full resolution of the big window.

The advantage of rendering everything at the same maximum resolution is: extremely accurate and quick mouse broadcasting, far better then anything else.
But it is more of a strain on system resources then other boxing software options.
Not really that big of a deal, definitely playable even on a modest system, especially with tweaks such as the virtual config files where slaves can have every option off, while the main has more options on.

Unless your graphic card is really old and outdated, graphically the only thing that is super challenging is shadows.
You can turn the shadow setting down, and a graphic card which is by far the worst part of the system will run just fine.
Most graphic options in wow challenge the processor more so then the graphic card -- things like spell effects, view distance, density of objects etc.

The largest advantage of multiple installs, is different settings on each install.
If you are using IS Boxer, you have the virtual configuration files, so one install is all you need.