I sure hope your fireplace is not an active one as that would be an unforunate spot to place the HDTV if it were.
Be aware when you compare HDTV and electronics prices, due to the market dynamics, it's not advised to buy an HDTV model until around end of January, which coincides with the Superbowl. After the SB, the last season's models will drop 30%. Also coinciding around that time is the CES (consumer electronics show) often held around mid Feb in Las Vegas. At that point all the new models get introduced. So prices on last season's model will be at the cheapest. This is how I made my purchase.
Regarding your cable provider going totally digital, in many countries (USA and UK) those entire markets have been totally digital for more than 2 or 3 years. Maybe in Canada it's different as I'm not aware of what your cable provider timeframe; however, if you stayed with your old TV, you'd have to add a converter that allows you to receive the HD digital signals as you're old TV won't pick up the transmission format. When you switch over to your new TV, it'll not be an issue as this capability is built in.
From your cable provider you'll also get a new cable box. you can program the TV to remember only the channels you want to watch. so each time you hunt for a show, you won't have to keep skipping over the ones you don't care about.
You won't have any trouble setting this function either. However, if you don't want to do it yourself you can ask the cable installer to do it when they set the new box up.
Be aware, your new cable box is different...it's not just that there are more channels. The new HDTV (used with new digital cable box), there'll be duplicate channels...one set for non HDTV programming for non HD tv watchers. The other set of channels (on a different spot higher up) are for the broadcasts in HD format. This is of no great concern but if the box has 500 channels. you'll probably only really watch the HD channels. there'll be such a pleasant experience- sort of like looking out a window..LOL
Electricity bills: depending on cost of electricity in your area, you'll save about $150-$200 from your old TV.
Viewing from 8 ft away an HDTV that is 42-inch to 46-inch is good to ideal but 52 in is overkill.
However, re use of Surround Sound to a living room is often times a worthwhile enhancement unless you jsut don't want to spend the money. I would not recommend a surround sound for a small bedroom, but for medium sized living room, with only a few exceptions. (narrow room), it will add a realistic enhancement and impact whatever you watch (except game shows like Jeopardy..LOL).