Why would you treat the subject of sex differently than any other? Treat it like any other subject---- just give her information she can understand, at whatever age she is, and save the rest for further questions later. You can't give "too much too early," because kids just stop listening after they get the answer they asked for. The problems begin if you are uncomfortable with discussing sex, as if there were something "wrong" with the subject, because then she becomes afraid to bring it up with you again when she needs more information, and asks her peers instead, or, even worse, relies on what she learns from TV...
And please don't just give her the "how babies are made" outline. Most 8 year olds have already picked up the mechanics of sex from the media. You have to make sure she understands the mechanics correctly, but most importantly, you have to teach her the more difficult , less tangible stuff, like how people in a sexual relationship should treat each other, the reasons why really young people should not get involved in sexual relationships, understanding of and respect for other sexual orientations, and the problems and dangers (as well as the delights) of sex. You will know you are doing a good job if she continues to ask questions as she matures.