C.,
Unless you think that there are developmental delays in your daughter, if you spend time enriching her envionment at home you are doing enough. If her preschool teachers think that there is something more, then they should refer her to the local school district for an evaluation. Ask them if they think an evaluation is in order, if they say no, then relax until she is in kindergarten and see how she does.
Pack this away as a possible warning for the future, but don't take it out unless you start to see her have trouble with something else later. If you have any other concerns about her development, then have her evaluated yourself.
Children develop at thier own pace, and children who enter kindergarten not knowing thier letters and numbers (who have no other developmental, lerning, or cognitive issues) will catch up with the rest of the children and they will all pretty much even out by the time they enter the 4th grade. That is why gifteness is difficult to identify in young children, becasue for some children, highly enriched enviornments produce a quick response and early learning of skills that cannot be sustained over the long haul.
You are probably better off reading with her everyday, talking to her with a nice vocabulary, and providing her the most enriched enviornment you can than you are using flash cards with her.