Mark, you should be just fine with the single frequency transducer.  You don't need the dual freq. unless you fish a lot of really deep water.  I have one, but I don't use the dual; instead I bought a single and installed on the opposite side of the boat.  The single (high) frequency transducer has a wider cone angle than the dual does, which I prefer.

Well I'm a more power the better guy,, I think the best thing for you to do and have a better understanding of depth finders / fish finders is go to both Lowrance's and hummingbird's web sites and read the how a fish finder works sections, they actually provide quite a bit of knowledge on things like power, freq, cone angles, arches and all kinds of good stuff.

Personally I like 4000 watts of power because it like comparing a pen light  to a Mag lite in a dark room, 200, 455, and 500 Khz seem to be the best freq for finding cats,,but a 50khz will also do the job,,,but your cone angle is sharply reduced so you have to be right on top of the fish to see em.  Dual freq or even triple freq units will use all the freq or just selected freq to aide you in finding your quarry,  from my experience 455/500 khz does good down to about 30 ft after that it becomes a little weak,  200 hz will get you down past a hundred feet and give you excellent bottom structure readings, 50 khz will be for really deep ocean fishing of several hundred feet.  hope this helps,,,personally I would drop the lowrance line and get a humminbird as most people find them easier to operate and use.