Hi Slash,
the weather in Aug will be similar to July - so check what the weather is like today, and expect something similar. It will be hot - about 35 degrees, and humid aswell. If your wife doesnt like the heat, it would be much more pleasant for her in sep.
Also in Sep, there will be less tourists in the restaurants etc, and the whole area less busy in general.
Having said that, Marbella is nice, with an old town with has loads of lovely outdoor restaurants and plenty of attractions nearby, so though I cant recommend a hotel, would recommend the destination.