Complete motor/tranny/wiring package usually goes for $2500 + shipping.

There are 2 S13 motors - the red top and the black top. The specs are the same for both. Best for upgrading according to several Nissan experts (I agree wholeheartedly).

The S14 motor's specs (This is also a black top) are the same but it comes with a T28.

The S15 black top comes with a T28 and Nissan's VTC. Stock it's mated with a 6 speed.

For more info:
www.heavythrottle.com
www.freshalloy.com