Honestly, my wow beta client (2 builds ago, however) ran like SHIT. When it was minimized my whole pc was lagging. The single copy used more than 400mb ram (no addons, current main wow instance uses 270mb) and the cpu used ranged between 30-40% while my main active instances uses 20-25% at the moment.

This worries me, to be honest. I'm hoping this is because of the beta client that still needs to be polished etc...