I never had a dog, Mummy was a cat person. I love all animals however and have kept the feline tradition alive. I can't conceive of not having a furry one around. I got my first kitten at the age of 6 and will never forget the feeling of being told, "Oh, you like that one? Well, you just take him on home. Happy birthday!" My mother had arranged the kitten visit with her "friend" and I had no idea it was a ploy for me to get my own little cat.
However, since your daughter is only 8, I would not volunteer to foster. No matter how mature she is, I'm not sure an 8 year old can really grasp the concept of foster (as in we're keeping it only until it is adopted by its "forever" family). If she is passionate about dogs the way I was with my kitten, you'd have to kill her to get it away from her.
I think exposing her to shelters and/or volunteering is a good way to go. Also, do you have any relatives/friends/neighbors who have dogs? Perhaps she could "dog sit" for them which would be a good way for you to gauge her patience/boredom level with the "work" side. Most animal lovers adore talking about their pets and I'm sure would be very helpful in sharing the happy/busy side of animal parenting with her. Also, head to the library/bookstore and start getting her books on dogs. Not just the pretty breed picture books but books on training, care, and all the ins and outs of dog ownership.
I think you are showing wonderful concern as a Mommy to research this issue. I don't see that you asking a question is an indicator that you are not 100% sure at all. The internet (and its message boards) are a very effective way to find out information. You might also go to the different dog breed/association web sites as they will have descriptions of breeds and links to breeders who can provide a lot more indepth information about specific animals. Good luck!