WRX seats drop right in.
You will be left with one extra bolt for each side. Keep in mind that the WRX seats sit several inches taller (but driver's side has a height adjustment).
They are much more comfortable than my old 1998 Legacy seats. Look cool too.
2002-2003 are the same, 2004 and newer are different.
You will need to transfer over the seat belt slip because the WRX buckle is different than older styles.
Do yourself a huge favor and buy them from a vehicle that has NOT been wrecked. The seatbelt strain can bend the slider rails. I had a nightmare time with mine because of this.
Ebay has good prices, but shipping can be high. they are much heavier than older seats, because there is more bolstering.
You will have unmatched seats, but you can also find rear WRX seats too. Remember that the WRX sedan seats are not split like the wagon seats, and they are supposed to fit pretty well, but they will be slightly higher in the back.
I am going to order some seat covers from Attention to Detail which will make my seats match again.
hope that helps.