Direct replacement. (What Corey said) Maximum replacement interval for 2001 and later 4 cyl engines is at 100,000 miles. But that doesn't preclude the need for periodic visual inspection in the mean time.
On my V6, I had to replace my plugs at 60k after a visual inspection. I bought Autolites from the part store. The plug gap is printed on a sticker in front of the radiator top as I open the hood on my '02.
I personally Prefer Ngk Iradium plugs myself..and If I remember right according to blueman and a few other duratec owners over on RPS...thats whats recommended for the 2.3l duratec...their about 7.99 a plug..and They work GREAT!...just did a tune up on mine the other day...took out the origional plugs which had 101k miles on them!