I would go ahead and definitely have her spayed before she goes into first heat.
If a dog is spayed before she goes into heat for the first time there is virtually no chance of her uterine or overian cancer and greatly reduces her chances of developing mammary cancer. Right now our Bichon (who is 20 yrs old) is dying of mammary cancer and its devestating.
All our other dogs have been spayed. Delaney Jade was spayed at 12 wks old a just a bit over 2 lbs... maybe close to 3. She was adopted from the shelter at 6 wks old and had to be returned there for the spay btwn 8-12 wks. She came through with flying colors. The only problem I had was trying to keep her still so she wouldn't rip her stitches. It did not make her mean. She loves everyone, especially kids. She is my "velcro dog" and doesn't like being more than a few feet from her mommy.
We adopted Schatzie from the shelter on a saturday and they did her spay on a Monday and allowed us to pick her up then. I can't say if she changed any since I didn't know her before. But she is one of the most loving dogs and will lick you to death. Both her and Laney play, play, play, play. Neither are at all fat, in fact we have to watch Laney and makes sure she keeps weight on her.
I have also fostered several dogs and all were done before 6mnths old and all bounced back. In fact Schatzie had the hardest time and she was a year old.
Heres a good link on the reasons why its good to spay
http://www.doghause.com/spay.asp