If you go to the Scott's website they have a handy little Lawn Care Plan section. You put in your address and zip code and it will tell you the size of your lawn and what type of product you should apply. It's pretty handy.
I bought a 5,000 lb bag of the Scotts Turf Builder Halts Crabgrass Preventer With Lawn Food for ~$25 and applied it Tuesday morning. I didn't really want to do it as early in the morning as I did, because the grass was still a little damp, but it was about the only time I could get it done when it wasn't going to rain for a couple days.
I already have a bag of Scotts Turf Builder Weed & Feed for later in the Spring (probably May/June-ish). Hopefully I won't forget.
I will then apply a third bag of feed with summer guard during the middle of summer, and one with winter guard in the fall.
Hopefully that will help the lawn look nice and green and be weed-free. Now I just need to get a new lawn mower...