From the Atkins questions answered thread:
18. How many calories should I be eating? How much fat? How much protein?
The lower your carb intake, the less you need to count calories. It's not a good idea to eat less than 1200 per day. If you're a woman then 1500 per day is a good average but there's no need to worry if you're over or under - this diet does not work like other diets do. You're burning fat so your body needs more calories.
How much fat at protein depends on your calorie intake for the day. You're aiming to get 65 percent of your daily calories from fat, 30 percent from protein, and 5 percent from carbs. Bear in mind there are 9 calories in a gram of fat compared to 4 calories in a gram of protein, so you don't need to eat pure fat to get to those percentages - cook with oil and butter, eat cheese and mayo and the skin on your roast chicken and do it guilt free. Enjoy.