Thetransducer that comes with the 798 is the Compact SI transducer and only has 455khz. The 997c and 1197c come with the HDSI larger transducer that has 455khz and 800khz. You will not be able to use the 800khz on the 798c, but you can use the 455khz.

I don't think the images are any better from what I have seen, but I dont have a 798c to test it with though.

You may get a better answer from the Humminbird Reps that visit the Side Imaging Forums.

Good Luck,

RGecy