Alright, what type of cable box do you have in the first place? I personally have the Motorola HD box from Comcast. I have yet to purchase a true HD-TV, but am also looking. Currently I have a Dell 2407 (24" 1920x1200 LCD) that I use for my computer and also for gaming on the x360 and for watching movies and such. I also have a set of logitech z5500s. I have the cable box connected to the monitor via digital coax and component. I have the xbox 360 connected via VGA and optical. My computer is connected via normal stereo analogue (despite having an older board with a soundstorm 5.1 chip on it - as I rarely do anything on the computer that doesn't involve me using my headphones). The sound works beautifully for everything. The logitechs will pretty much play anything that you put into them, and your new TV should have optical S/PDIF out or digital coax out (the TV you linked has both). Some TVs will pass through signals, and some will only output certain formats. Ideally, you would just want the signal passed through to your speakers... IE: not screwed with in any way (as per my understanding). I know I've seen some sets that only output in stereo despite using an optical output. You wouldn't want to be putting a 5.1 channel source into said TV (say from an HD-DVD player, PS3, 360, etc...) and have it cut down to stereo before being passed to your speakers (which would then do their best to emulate a 5.1 channel signal).
In all honesty, if I were in your situation, I would go for a bit less of a TV and spend a bit on a nice pair of HT speakers. Don't get me wrong, I love my z5500s, they are great computer speakers, but I'm not so sure that I would drop 3k on a TV and then be using my computer speakers with it. To each his own though, I'm sure you will love that LG!