OK, after doing some more experimenting and reading all the manuals, the only way I will be able to pass 5.1 sound is through an SPDIF cable. I tried using the dongle and HDMI cable, but it will only pass stereo sound, because my receiver has no HDMI inputs and the TV will only pass through stereo, not 5.1.
However, I've discovered something in the process. From 6-8 feet away, the apparent screen size of the 46" is about the same as my 24" on my desktop from 2.5 feet away.
That is, it takes up about the same angle of vision. It doesn't seem larger to the eye from the viewing distance I'd be using. Now, if I was to put a 32 or 37" on my desktop, that would be a different story. And my desk chair is way more comfortable than leaning forward on the couch to type on a table.
So, while I might consider doing this regularly if I had a 19" monitor, I won't be doing this for hooking up my main machine. Learned some stuff about my contemplated HTPC build though.
Thanks again for the help guys. Oh, and Anitec.ca has some cheaper toslink SPDIF cables for those of you who might be looking.