I've made three batches of fruit ales. The first was with extract. It was just OK. The second was with blueberries. I used a little over two pounds of them (five gallon batches). This came out nice, more like a hint than a real strong flavoring. My current batch is raspberry again, but this time using actual raspberries. I used about four pounds, and the flavor is very good - beery but also a strong fruit taste. Raspberries are pretty versatile and go great in beer.
So you have to decide how strong you would like the fruit taste to be. Three pounds might be a nice middle ground.
This is how I've done it. I've pasteurized my whole fruit - put in a pot with some water, enough to cover them, and heat them to just short of a boil, break the skins, then add them into the secondary. The cut off top of a two liter soda bottle works great as a funnel for this. I never stirred anything, and I in general don't think you should intervene in your brew process any more than is absolutely necessary.
I let them sit in there for a week (you will see how washed out looking the fruit gets), then rerack to a tertiary and let it clear for another week or two.
For the extract batch, I just added the extract into the bottling bucket right out of the bottle. No color from extract I used though, which is one of the things I like about using real fruit.
Finally, if you have not read over this thread, do so - lots of good info here: