There is a lot of mis-information on this topic.

The standard transducer in the box from the Humminbird factory for the 898c SI and 998c SI is the XHS 9 HDSI 180 T and the standard GPS antenna in the box from Humminbird is the GR-50.
Universal Mania won't change those items to sell for a cheaper price.

The 798ci HD SI is shipped with the standard Compact SI transducer for the transom--- XNT 9 SI 180 T. In no case is the larger HDSI transducer shipped with that model unless the dealer does the swap and you pay the difference.
The HDSI transducer is the standard one for the larger SI units.

The HD in the model number of the 798ci HD SI only means it is the 2011 year model and has Ethernet capability. The rest of the features are the same as the previous model 798ci SI.
HD is basically marketing the high pixel count of the smaller screens which is the highest in industry. None of the larger screen models have the pixel density of the 798 which is 31, 170.5 per square inch. The larger the display, the less dense the pixels per square inch they have with the 1198c SI being the lowest at 8,858.2

The HD in the XHS 9 HDSI 180 T transducer model name means it functions with the 455 and 800 kHz Side Imaging frequencies of the larger display models.
All the 798 models use just 455 kHz for Side Imaging.

If you want the HDSI transducer for your 798, you can exchange it through Humminbird and pay the cost difference plus shipping. MSRP for the HDSI transducer is about $270. MSRP for the Compact SI transducer is $185.