We had mill pond conditions in January and a force 10 in June. In August you should be ok but this year has been less predictable than others. My advice is to look at or listen to the forecasts (BBC weather on the net) up to 10 days before and then you will get some kind of indication. Main thing is not worry about it, she's a big beautiful ship that takes a lot to tilt it.
Just got back from NOS & on way out was very rough. Was glad id bought the sea-bands but way back although wind was high we were going with it so hardly felt a thing. Just remember to take your sea sickness tablets just in case.
Cruised through the Bay of Biscay in September last year and it was like a 'mill pond', cruised again in the Bay in December 2006 and it was very rough. I guess you pays you money and takes your chances, but would imagine it would be OK in August.
Cruised the Bay of Biscay twice last year with Sea Princess in July. Was bouncy on the way down to the Med but not unpleasant. Much rougher on way back with quite a few people succumbing to sickness. I took Stugeron just to be on the safe side - could have made a mint selling tablets! Personally I don't think you can predict it - just go with what you get but take some medication with you just in case..