I strongly suggest the
Parallax PING))) sensor. 1024 has used these for a while now with great success. You can
use my driver for them if you'd like.
As a side note, about half way through this season I will be releasing the 2008 version of the PING))) driver which supports simultaneous, asynchronous measurement from four PING))) modules using a single hardware timer and four I/O pins, plus it has high efficiency interrupts and an easy to understand interface. Two encoders can still be operated in parallel with the four ultrasonic sensors.
-q